Buggy python submission (verdict: Time Limit Exceeded):
s = input()
n = len(s)
p = 10 ** 9 + 7

# i = 0, ..., nについて
# dp[i][0] = i文字目まででありうる文字列の数（ABCなら1、AB?なら3）
# dp[i][1] = i文字目までに含まれるAの数
# dp[i][2] = i文字目までに含まれるABのパターン数
# dp[i][3] = i文字目までに含まれるABCのパターン数
dp = [[0 for j in range(4)] for i in range(n + 1)]
dp[0][0] = 1 # ''空の文字列は1パターン
for j in range(1, 4):
    dp[0][j] = 0  # 空の文字列にはAもABもABCも入っていない

for i in range(1, n + 1):
    m = 3 if s[i - 1] == '?' else 1

    for j in range(4):
        dp[i][j] = dp[i - 1][j] * m

    if s[i - 1] == 'A' or s[i - 1] == '?':
        dp[i][1] =  (dp[i][1] + dp[i - 1][0]) % p
    if s[i - 1] == 'B' or s[i - 1] == '?':
        dp[i][2] = (dp[i][2] + dp[i - 1][1]) % p
    if s[i - 1] == 'C' or s[i - 1] == '?':
        dp[i][3] = (dp[i][3] + dp[i - 1][2]) % p

print(dp[n][3])


Fixed submission:
s = input()
n = len(s)
p = 10 ** 9 + 7

q = s.count('?')
l_a = [0] * n  # l_a[i] := s[i]より左にあるAの数
l_x = [0] * n  # l_x[i] := s[i]より左にある?の数
r_c = [0] * n  # r_c[i] := s[i]より右にあるCの数
r_x = [0] * n  # r_x[i] := s[i]より右にある?の数

for i in range(1, n):  # 1〜n-1
    l_a[i] = l_a[i - 1] + int(s[i - 1] == 'A')
    l_x[i] = l_x[i - 1] + int(s[i - 1] == '?')
    r_c[n - i - 1] = r_c[n - i] + int(s[n - i] == 'C')
    r_x[n - i - 1] = r_x[n - i] + int(s[n - i] == '?')

ans = 0
pow_q = []
for i in range(0, 4):
    pow_q.append(max(1, 3 ** (q - i) % p))

for i in range(n):
    if s[i] == 'B':
        ans += (l_a[i] * r_c[i] % p) * pow_q[0] % p  # ABC
        ans += (l_x[i] * r_c[i] % p) * pow_q[1] % p # ?BC
        ans += (l_a[i] * r_x[i] % p) * pow_q[1] % p # AB?
        ans += (l_x[i] * r_x[i] % p) * pow_q[2] % p # ?B?
        ans %= p
    elif s[i] == '?':
        ans += (l_a[i] * r_c[i] % p) * pow_q[1] % p # A?C
        ans += (l_x[i] * r_c[i] % p) * pow_q[2] % p # ??C
        ans += (l_a[i] * r_x[i] % p) * pow_q[2] % p # A??
        ans += (l_x[i] * r_x[i] % p) * pow_q[3] % p # ???
        ans %= p

print(ans)

Buggy python submission (verdict: Runtime Error):
s = input()
n = len(s)
p = 10 ** 9 + 7

# i = 0, ..., nについて
# dp[i][0] = i文字目まででありうる文字列の数（ABCなら1、AB?なら3）
# dp[i][1] = i文字目までに含まれるAの数
# dp[i][2] = i文字目までに含まれるABのパターン数
# dp[i][3] = i文字目までに含まれるABCのパターン数
dp = [[0 for j in range(4)] for i in range(n + 1)]
dp[0][0] = 1 # ''空の文字列は1パターン
for j in range(1, 4):
    dp[0][j] = 0  # 空の文字列にはAもABもABCも入っていない

for i in range(1, n + 1):
    m = 3 if s[i - 1] == '?' else 1

    for j in range(4):
        dp[i][j] = dp[i - 1][j] * m

    if s[i - 1] == 'A' or s[i - 1] == '?':
        dp[i][A] =  (dp[i][A] + dp[i - 1][0]) % p
    if s[i - 1] == 'B' or s[i - 1] == '?':
        dp[i][AB] = (dp[i][AB] + dp[i - 1][A]) % p
    if s[i - 1] == 'C' or s[i - 1] == '?':
        dp[i][ABC] = (dp[i][ABC] + dp[i - 1][AB]) % p

print(dp[n][ABC])

Fixed submission:
s = input()
n = len(s)
p = 10 ** 9 + 7

q = s.count('?')
l_a = [0] * n  # l_a[i] := s[i]より左にあるAの数
l_x = [0] * n  # l_x[i] := s[i]より左にある?の数
r_c = [0] * n  # r_c[i] := s[i]より右にあるCの数
r_x = [0] * n  # r_x[i] := s[i]より右にある?の数

for i in range(1, n):  # 1〜n-1
    l_a[i] = l_a[i - 1] + int(s[i - 1] == 'A')
    l_x[i] = l_x[i - 1] + int(s[i - 1] == '?')
    r_c[n - i - 1] = r_c[n - i] + int(s[n - i] == 'C')
    r_x[n - i - 1] = r_x[n - i] + int(s[n - i] == '?')

ans = 0
pow_q = []
for i in range(0, 4):
    pow_q.append(max(1, 3 ** (q - i) % p))

for i in range(n):
    if s[i] == 'B':
        ans += (l_a[i] * r_c[i] % p) * pow_q[0] % p  # ABC
        ans += (l_x[i] * r_c[i] % p) * pow_q[1] % p # ?BC
        ans += (l_a[i] * r_x[i] % p) * pow_q[1] % p # AB?
        ans += (l_x[i] * r_x[i] % p) * pow_q[2] % p # ?B?
        ans %= p
    elif s[i] == '?':
        ans += (l_a[i] * r_c[i] % p) * pow_q[1] % p # A?C
        ans += (l_x[i] * r_c[i] % p) * pow_q[2] % p # ??C
        ans += (l_a[i] * r_x[i] % p) * pow_q[2] % p # A??
        ans += (l_x[i] * r_x[i] % p) * pow_q[3] % p # ???
        ans %= p

print(ans)

Buggy python submission (verdict: Runtime Error):
s = input()
n = len(s)
p = 10 ** 9 + 7

# i = 0, ..., nについて
# dp[i][0] = i文字目まででありうる文字列の数（ABCなら1、AB?なら3）
# dp[i][1] = i文字目までに含まれるAの数
# dp[i][2] = i文字目までに含まれるABのパターン数
# dp[i][3] = i文字目までに含まれるABCのパターン数
dp = [[0 for j in range(4)] for i in range(n + 1)]
dp[0][0] = 1 # ''空の文字列は1パターン
for j in range(1, 4):
    dp[0][j] = 0  # 空の文字列にはAもABもABCも入っていない

print("dp[0] = {}".format(dp[0]))
for i in range(1, n + 1):
    m = 3 if s[i - 1] == '?' else 1

    for j in range(4):
        dp[i][j] = dp[i - 1][j] * m

    if s[i - 1] == 'A' or s[i - 1] == '?':
        dp[i][A] =  (dp[i][A] + dp[i - 1][0]) % p
    if s[i - 1] == 'B' or s[i - 1] == '?':
        dp[i][AB] = (dp[i][AB] + dp[i - 1][A]) % p
    if s[i - 1] == 'C' or s[i - 1] == '?':
        dp[i][ABC] = (dp[i][ABC] + dp[i - 1][AB]) % p

print(dp[n][ABC])

Fixed submission:
s = input()
n = len(s)
p = 10 ** 9 + 7

q = s.count('?')
l_a = [0] * n  # l_a[i] := s[i]より左にあるAの数
l_x = [0] * n  # l_x[i] := s[i]より左にある?の数
r_c = [0] * n  # r_c[i] := s[i]より右にあるCの数
r_x = [0] * n  # r_x[i] := s[i]より右にある?の数

for i in range(1, n):  # 1〜n-1
    l_a[i] = l_a[i - 1] + int(s[i - 1] == 'A')
    l_x[i] = l_x[i - 1] + int(s[i - 1] == '?')
    r_c[n - i - 1] = r_c[n - i] + int(s[n - i] == 'C')
    r_x[n - i - 1] = r_x[n - i] + int(s[n - i] == '?')

ans = 0
pow_q = []
for i in range(0, 4):
    pow_q.append(max(1, 3 ** (q - i) % p))

for i in range(n):
    if s[i] == 'B':
        ans += (l_a[i] * r_c[i] % p) * pow_q[0] % p  # ABC
        ans += (l_x[i] * r_c[i] % p) * pow_q[1] % p # ?BC
        ans += (l_a[i] * r_x[i] % p) * pow_q[1] % p # AB?
        ans += (l_x[i] * r_x[i] % p) * pow_q[2] % p # ?B?
        ans %= p
    elif s[i] == '?':
        ans += (l_a[i] * r_c[i] % p) * pow_q[1] % p # A?C
        ans += (l_x[i] * r_c[i] % p) * pow_q[2] % p # ??C
        ans += (l_a[i] * r_x[i] % p) * pow_q[2] % p # A??
        ans += (l_x[i] * r_x[i] % p) * pow_q[3] % p # ???
        ans %= p

print(ans)

Buggy python submission (verdict: Time Limit Exceeded):
import sys
input = sys.stdin.readline
s = input()
n = len(s)
p = 10 ** 9 + 7

q = s.count('?')
l_a = [0] * n  # l_a[i] := s[i]より左にあるAの数
l_x = [0] * n  # l_x[i] := s[i]より左にある?の数
r_c = [0] * n  # r_c[i] := s[i]より右にあるCの数
r_x = [0] * n  # r_x[i] := s[i]より右にある?の数

for i in range(1, n):  # 1〜n-1
    l_a[i] = l_a[i - 1] + int(s[i - 1] == 'A')
    l_x[i] = l_x[i - 1] + int(s[i - 1] == '?')
    r_c[n - i - 1] = r_c[n - i] + int(s[n - i] == 'C')
    r_x[n - i - 1] = r_x[n - i] + int(s[n - i] == '?')

ans = 0
for i in range(n):
    if s[i] == 'B':
        ans += (l_a[i] * r_c[i] % p) * (3 ** q % p) % p       # ABC
        if q >= 1:
            ans += (l_x[i] * r_c[i] % p) * (3 ** (q - 1) % p) % p # ?BC
            ans += (l_a[i] * r_x[i] % p) * (3 ** (q - 1) % p) % p # AB?
        if q >= 2:
            ans += (l_x[i] * r_x[i] % p) * (3 ** (q - 2) % p) % p # ?B?
        ans %= p
    elif s[i] == '?':
        if q >= 1:
            ans += (l_a[i] * r_c[i] % p) * (3 ** (q - 1) % p) % p # A?C
        if q >= 2:
            ans += (l_x[i] * r_c[i] % p) * (3 ** (q - 2) % p) % p # ??C
            ans += (l_a[i] * r_x[i] % p) * (3 ** (q - 2) % p) % p # A??
        if q >= 3:
            ans += (l_x[i] * r_x[i] % p) * (3 ** (q - 3) % p) % p # ???
        ans %= p

print(ans)

Fixed submission:
s = input()
n = len(s)
p = 10 ** 9 + 7

q = s.count('?')
l_a = [0] * n  # l_a[i] := s[i]より左にあるAの数
l_x = [0] * n  # l_x[i] := s[i]より左にある?の数
r_c = [0] * n  # r_c[i] := s[i]より右にあるCの数
r_x = [0] * n  # r_x[i] := s[i]より右にある?の数

for i in range(1, n):  # 1〜n-1
    l_a[i] = l_a[i - 1] + int(s[i - 1] == 'A')
    l_x[i] = l_x[i - 1] + int(s[i - 1] == '?')
    r_c[n - i - 1] = r_c[n - i] + int(s[n - i] == 'C')
    r_x[n - i - 1] = r_x[n - i] + int(s[n - i] == '?')

ans = 0
pow_q = []
for i in range(0, 4):
    pow_q.append(max(1, 3 ** (q - i) % p))

for i in range(n):
    if s[i] == 'B':
        ans += (l_a[i] * r_c[i] % p) * pow_q[0] % p  # ABC
        ans += (l_x[i] * r_c[i] % p) * pow_q[1] % p # ?BC
        ans += (l_a[i] * r_x[i] % p) * pow_q[1] % p # AB?
        ans += (l_x[i] * r_x[i] % p) * pow_q[2] % p # ?B?
        ans %= p
    elif s[i] == '?':
        ans += (l_a[i] * r_c[i] % p) * pow_q[1] % p # A?C
        ans += (l_x[i] * r_c[i] % p) * pow_q[2] % p # ??C
        ans += (l_a[i] * r_x[i] % p) * pow_q[2] % p # A??
        ans += (l_x[i] * r_x[i] % p) * pow_q[3] % p # ???
        ans %= p

print(ans)

Buggy python submission (verdict: Time Limit Exceeded):
s = input()
n = len(s)
p = 10 ** 9 + 7
 
q = s.count('?')
l_a = [0] * n  # l_a[i] := s[i]より左にあるAの数
l_x = [0] * n  # l_x[i] := s[i]より左にある?の数
r_c = [0] * n  # r_c[i] := s[i]より右にあるCの数
r_x = [0] * n  # r_x[i] := s[i]より右にある?の数
 
for i in range(1, n):  # 1〜n-1
    l_a[i] = l_a[i - 1] + int(s[i - 1] == 'A')
    l_x[i] = l_x[i - 1] + int(s[i - 1] == '?')
    r_c[n - i - 1] = r_c[n - i] + int(s[n - i] == 'C')
    r_x[n - i - 1] = r_x[n - i] + int(s[n - i] == '?')
 
ans = 0
for i in range(n):
    if s[i] == 'B':
        ans += (l_a[i] * r_c[i] % p) * (3 ** q % p) % p       # ABC
        if q >= 1:
            ans += (l_x[i] * r_c[i] % p) * (3 ** (q - 1) % p) % p # ?BC
            ans += (l_a[i] * r_x[i] % p) * (3 ** (q - 1) % p) % p # AB?
        if q >= 2:
            ans += (l_x[i] * r_x[i] % p) * (3 ** (q - 2) % p) % p # ?B?
        ans %= p
    elif s[i] == '?':
        if q >= 1:
            ans += (l_a[i] * r_c[i] % p) * (3 ** (q - 1) % p) % p # A?C
        if q >= 2:
            ans += (l_x[i] * r_c[i] % p) * (3 ** (q - 2) % p) % p # ??C
            ans += (l_a[i] * r_x[i] % p) * (3 ** (q - 2) % p) % p # A??
        if q >= 3:
            ans += (l_x[i] * r_x[i] % p) * (3 ** (q - 3) % p) % p # ???
        ans %= p
 
print(ans)

Fixed submission:
s = input()
n = len(s)
p = 10 ** 9 + 7

q = s.count('?')
l_a = [0] * n  # l_a[i] := s[i]より左にあるAの数
l_x = [0] * n  # l_x[i] := s[i]より左にある?の数
r_c = [0] * n  # r_c[i] := s[i]より右にあるCの数
r_x = [0] * n  # r_x[i] := s[i]より右にある?の数

for i in range(1, n):  # 1〜n-1
    l_a[i] = l_a[i - 1] + int(s[i - 1] == 'A')
    l_x[i] = l_x[i - 1] + int(s[i - 1] == '?')
    r_c[n - i - 1] = r_c[n - i] + int(s[n - i] == 'C')
    r_x[n - i - 1] = r_x[n - i] + int(s[n - i] == '?')

ans = 0
pow_q = []
for i in range(0, 4):
    pow_q.append(max(1, 3 ** (q - i) % p))

for i in range(n):
    if s[i] == 'B':
        ans += (l_a[i] * r_c[i] % p) * pow_q[0] % p  # ABC
        ans += (l_x[i] * r_c[i] % p) * pow_q[1] % p # ?BC
        ans += (l_a[i] * r_x[i] % p) * pow_q[1] % p # AB?
        ans += (l_x[i] * r_x[i] % p) * pow_q[2] % p # ?B?
        ans %= p
    elif s[i] == '?':
        ans += (l_a[i] * r_c[i] % p) * pow_q[1] % p # A?C
        ans += (l_x[i] * r_c[i] % p) * pow_q[2] % p # ??C
        ans += (l_a[i] * r_x[i] % p) * pow_q[2] % p # A??
        ans += (l_x[i] * r_x[i] % p) * pow_q[3] % p # ???
        ans %= p

print(ans)

Buggy python submission (verdict: Time Limit Exceeded):
s = input()
n = len(s)
p = 10 ** 9 + 7

q = s.count('?')
l_a = [0] * n  # l_a[i] := s[i]より左にあるAの数
l_x = [0] * n  # l_x[i] := s[i]より左にある?の数
r_c = [0] * n  # r_c[i] := s[i]より右にあるCの数
r_x = [0] * n  # r_x[i] := s[i]より右にある?の数

for i in range(1, n):  # 1〜n-1
    l_a[i] = l_a[i - 1] + int(s[i - 1] == 'A')
    l_x[i] = l_x[i - 1] + int(s[i - 1] == '?')
    r_c[n - i - 1] = r_c[n - i] + int(s[n - i] == 'C')
    r_x[n - i - 1] = r_x[n - i] + int(s[n - i] == '?')

ans = 0
for i in range(n):
    if s[i] == 'B':
        ans += (l_a[i] * r_c[i] % p) * (3 ** q % p) % p       # ABC
        if q >= 1:
            ans += (l_x[i] * r_c[i] % p) * (3 ** (q - 1) % p) % p # ?BC
            ans += (l_a[i] * r_x[i] % p) * (3 ** (q - 1) % p) % p # AB?
        if q >= 2:
            ans += (l_x[i] * r_x[i] % p) * (3 ** (q - 2) % p) % p # ?B?
        ans %= p
    elif s[i] == '?':
        if q >= 1:
            ans += (l_a[i] * r_c[i] % p) * (3 ** (q - 1) % p) % p # A?C
        if q >= 2:
            ans += (l_x[i] * r_c[i] % p) * (3 ** (q - 2) % p) % p # ??C
            ans += (l_a[i] * r_x[i] % p) * (3 ** (q - 2) % p) % p # A??
        if q >= 3:
            ans += (l_x[i] * r_x[i] % p) * (3 ** (q - 3) % p) % p # ???
        ans %= p

print(ans)


Fixed submission:
s = input()
n = len(s)
p = 10 ** 9 + 7

q = s.count('?')
l_a = [0] * n  # l_a[i] := s[i]より左にあるAの数
l_x = [0] * n  # l_x[i] := s[i]より左にある?の数
r_c = [0] * n  # r_c[i] := s[i]より右にあるCの数
r_x = [0] * n  # r_x[i] := s[i]より右にある?の数

for i in range(1, n):  # 1〜n-1
    l_a[i] = l_a[i - 1] + int(s[i - 1] == 'A')
    l_x[i] = l_x[i - 1] + int(s[i - 1] == '?')
    r_c[n - i - 1] = r_c[n - i] + int(s[n - i] == 'C')
    r_x[n - i - 1] = r_x[n - i] + int(s[n - i] == '?')

ans = 0
pow_q = []
for i in range(0, 4):
    pow_q.append(max(1, 3 ** (q - i) % p))

for i in range(n):
    if s[i] == 'B':
        ans += (l_a[i] * r_c[i] % p) * pow_q[0] % p  # ABC
        ans += (l_x[i] * r_c[i] % p) * pow_q[1] % p # ?BC
        ans += (l_a[i] * r_x[i] % p) * pow_q[1] % p # AB?
        ans += (l_x[i] * r_x[i] % p) * pow_q[2] % p # ?B?
        ans %= p
    elif s[i] == '?':
        ans += (l_a[i] * r_c[i] % p) * pow_q[1] % p # A?C
        ans += (l_x[i] * r_c[i] % p) * pow_q[2] % p # ??C
        ans += (l_a[i] * r_x[i] % p) * pow_q[2] % p # A??
        ans += (l_x[i] * r_x[i] % p) * pow_q[3] % p # ???
        ans %= p

print(ans)