Buggy python submission (verdict: Runtime Error):
S = input()
l = len(S)
A,B,C,q = [0]*l,[0]*l,[0]*l,[0]*l
inf = 1000000007
for i in range(l):
    if S[i] == 'A':
        A[i] = 1
    elif S[i] == 'B':
        B[i] = 1
    elif S[i] == 'C':
        C[i] = 1
    else:
        q[i] = 1
    A[]

for i in range(l-1):
    A[i+1] += A[i]
    B[i+1] += B[i]
    C[i+1] += C[i]
    q[i+1] += q[i]

ans = 0
for i in range(1,l-1):
    if S[i] == 'B':
        ans += (A[i-1]) * (C[-1]-C[i]) * (3**q[-1])%inf
        ans %= inf
        if q[-1] >= 1:
            ans += (q[i-1]) * (C[-1] - C[i]) * (3**(q[-1]-1)%inf)
            ans %= inf
            ans += (A[i-1]) * (q[-1] - q[i]) * (3**(q[-1]-1)%inf)
            ans %= inf
        if q[-1] >= 2:
            ans += (q[i-1]) * (q[-1] - q[i]) * (3**(q[-1]-2)%inf)
            ans %= inf

    elif S[i] == '?':
        ans += (A[i-1]) * (C[-1]-C[i]) * (3**(q[-1]-1)%inf)
        ans %= inf
        if q[-1] >= 2:
            ans += (q[i-1]) * (C[-1] - C[i]) * (3**(q[-1]-2)%inf)
            ans %= inf
            ans += (A[i-1]) * (q[-1] - q[i]) * (3**(q[-1]-2)%inf)
            ans %= inf
        if q[-1] >= 3:
            ans += (q[i-1]) * (q[-1] - q[i]) * (3**(q[-1]-3)%inf)
            ans %= inf

print(ans%inf)


 

Fixed submission:
S = input()
l = len(S)
A,B,C,q = [0]*l,[0]*l,[0]*l,[0]*l
inf = 1000000007
for i in range(l):
    if S[i] == 'A':
        A[i] = 1
    elif S[i] == 'B':
        B[i] = 1
    elif S[i] == 'C':
        C[i] = 1
    else:
        q[i] = 1

for i in range(l-1):
    A[i+1] += A[i]
    B[i+1] += B[i]
    C[i+1] += C[i]
    q[i+1] += q[i]

ans = 0
for i in range(1,l-1):
    if S[i] == 'B':
        ans += (A[i-1]) * (C[-1]-C[i]) * pow(3,q[-1],inf)
        ans %= inf
        if q[-1] >= 1:
            ans += (q[i-1]) * (C[-1] - C[i]) * pow(3,q[-1]-1,inf)
            ans %= inf
            ans += (A[i-1]) * (q[-1] - q[i]) * pow(3,q[-1]-1,inf)
            ans %= inf
        if q[-1] >= 2:
            ans += (q[i-1]) * (q[-1] - q[i]) * pow(3,q[-1]-2,inf)
            ans %= inf

    elif S[i] == '?':
        ans += (A[i-1]) * (C[-1]-C[i]) * pow(3,q[-1]-1,inf)
        ans %= inf
        if q[-1] >= 2:
            ans += (q[i-1]) * (C[-1] - C[i]) * pow(3,q[-1]-2,inf)
            ans %= inf
            ans += (A[i-1]) * (q[-1] - q[i]) * pow(3,q[-1]-2,inf)
            ans %= inf
        if q[-1] >= 3:
            ans += (q[i-1]) * (q[-1] - q[i]) * pow(3,q[-1]-3,inf)
            ans %= inf

print(ans%inf)

Buggy python submission (verdict: Time Limit Exceeded):
S = input()
l = len(S)
A,B,C,q = [0]*l,[0]*l,[0]*l,[0]*l
inf = 1000000007
for i in range(l):
    if S[i] == 'A':
        A[i] = 1
    elif S[i] == 'B':
        B[i] = 1
    elif S[i] == 'C':
        C[i] = 1
    else:
        q[i] = 1

for i in range(l-1):
    A[i+1] += A[i]
    B[i+1] += B[i]
    C[i+1] += C[i]
    q[i+1] += q[i]

ans = 0
for i in range(1,l-1):
    if S[i] == 'B':
        ans += (A[i-1]) * (C[-1]-C[i]) * (3**q[-1])
        ans %= inf
        if q[-1] >= 1:
            ans += (q[i-1]) * (C[-1] - C[i]) * (3**(q[-1]-1))
            ans %= inf
            ans += (A[i-1]) * (q[-1] - q[i]) * (3**(q[-1]-1))
            ans %= inf
        if q[-1] >= 2:
            ans += (q[i-1]) * (q[-1] - q[i]) * (3**(q[-1]-2))
            ans %= inf

    elif S[i] == '?':
        ans += (A[i-1]) * (C[-1]-C[i]) * (3**(q[-1]-1))
        ans %= inf
        if q[-1] >= 2:
            ans += (q[i-1]) * (C[-1] - C[i]) * (3**(q[-1]-2))
            ans %= inf
            ans += (A[i-1]) * (q[-1] - q[i]) * (3**(q[-1]-2))
            ans %= inf
        if q[-1] >= 3:
            ans += (q[i-1]) * (q[-1] - q[i]) * (3**(q[-1]-3))
            ans %= inf

print(ans%inf)


 

Fixed submission:
S = input()
l = len(S)
A,B,C,q = [0]*l,[0]*l,[0]*l,[0]*l
inf = 1000000007
for i in range(l):
    if S[i] == 'A':
        A[i] = 1
    elif S[i] == 'B':
        B[i] = 1
    elif S[i] == 'C':
        C[i] = 1
    else:
        q[i] = 1

for i in range(l-1):
    A[i+1] += A[i]
    B[i+1] += B[i]
    C[i+1] += C[i]
    q[i+1] += q[i]

ans = 0
for i in range(1,l-1):
    if S[i] == 'B':
        ans += (A[i-1]) * (C[-1]-C[i]) * pow(3,q[-1],inf)
        ans %= inf
        if q[-1] >= 1:
            ans += (q[i-1]) * (C[-1] - C[i]) * pow(3,q[-1]-1,inf)
            ans %= inf
            ans += (A[i-1]) * (q[-1] - q[i]) * pow(3,q[-1]-1,inf)
            ans %= inf
        if q[-1] >= 2:
            ans += (q[i-1]) * (q[-1] - q[i]) * pow(3,q[-1]-2,inf)
            ans %= inf

    elif S[i] == '?':
        ans += (A[i-1]) * (C[-1]-C[i]) * pow(3,q[-1]-1,inf)
        ans %= inf
        if q[-1] >= 2:
            ans += (q[i-1]) * (C[-1] - C[i]) * pow(3,q[-1]-2,inf)
            ans %= inf
            ans += (A[i-1]) * (q[-1] - q[i]) * pow(3,q[-1]-2,inf)
            ans %= inf
        if q[-1] >= 3:
            ans += (q[i-1]) * (q[-1] - q[i]) * pow(3,q[-1]-3,inf)
            ans %= inf

print(ans%inf)

Buggy python submission (verdict: Time Limit Exceeded):
S = input()
l = len(S)
A,B,C,q = [0]*l,[0]*l,[0]*l,[0]*l
inf = 1000000007
for i in range(l):
    if S[i] == 'A':
        A[i] = 1
    elif S[i] == 'B':
        B[i] = 1
    elif S[i] == 'C':
        C[i] = 1
    else:
        q[i] = 1

for i in range(l-1):
    A[i+1] += A[i]
    B[i+1] += B[i]
    C[i+1] += C[i]
    q[i+1] += q[i]

ans = 0
for i in range(1,l-1):
    if S[i] == 'B':
        ans += (A[i-1]) * (C[-1]-C[i]) * (3**q[-1])
        ans %= inf
        if q[-1] >= 1:
            ans += (q[i-1]) * (C[-1] - C[i]) * (3**(q[-1]-1))
            ans %= inf
            ans += (A[i-1]) * (q[-1] - q[i]) * (3**(q[-1]-1))
            ans %= inf
        if q[-1] >= 2:
            ans += (q[i-1]) * (q[-1] - q[i]) * (3**(q[-1]-2))
            ans %= inf

    elif S[i] == '?':
        ans += (A[i-1]) * (C[-1]-C[i]) * (3**(q[-1]-1))
        ans %= inf
        if q[-1] >= 2:
            ans += (q[i-1]) * (C[-1] - C[i]) * (3**(q[-1]-2))
            ans %= inf
            ans += (A[i-1]) * (q[-1] - q[i]) * (3**(q[-1]-2))
            ans %= inf
        if q[-1] >= 3:
            ans += (q[i-1]) * (q[-1] - q[i]) * (3**(q[-1]-3))
            ans %= inf

print(ans%inf)


 

Fixed submission:
S = input()
l = len(S)
A,B,C,q = [0]*l,[0]*l,[0]*l,[0]*l
inf = 1000000007
for i in range(l):
    if S[i] == 'A':
        A[i] = 1
    elif S[i] == 'B':
        B[i] = 1
    elif S[i] == 'C':
        C[i] = 1
    else:
        q[i] = 1

for i in range(l-1):
    A[i+1] += A[i]
    B[i+1] += B[i]
    C[i+1] += C[i]
    q[i+1] += q[i]

ans = 0
for i in range(1,l-1):
    if S[i] == 'B':
        ans += (A[i-1]) * (C[-1]-C[i]) * pow(3,q[-1],inf)
        ans %= inf
        if q[-1] >= 1:
            ans += (q[i-1]) * (C[-1] - C[i]) * pow(3,q[-1]-1,inf)
            ans %= inf
            ans += (A[i-1]) * (q[-1] - q[i]) * pow(3,q[-1]-1,inf)
            ans %= inf
        if q[-1] >= 2:
            ans += (q[i-1]) * (q[-1] - q[i]) * pow(3,q[-1]-2,inf)
            ans %= inf

    elif S[i] == '?':
        ans += (A[i-1]) * (C[-1]-C[i]) * pow(3,q[-1]-1,inf)
        ans %= inf
        if q[-1] >= 2:
            ans += (q[i-1]) * (C[-1] - C[i]) * pow(3,q[-1]-2,inf)
            ans %= inf
            ans += (A[i-1]) * (q[-1] - q[i]) * pow(3,q[-1]-2,inf)
            ans %= inf
        if q[-1] >= 3:
            ans += (q[i-1]) * (q[-1] - q[i]) * pow(3,q[-1]-3,inf)
            ans %= inf

print(ans%inf)