Buggy python submission (verdict: Time Limit Exceeded):
import sys
INF = 10 ** 15
MOD = 10 ** 9 + 7
sys.setrecursionlimit(100000000)
input = sys.stdin.readline
from math import pi
from functools import lru_cache

def main():
    n = int(input())
    num = list(map(int,input().split()))
    
    @lru_cache(maxsize = None)
    def dfs(a,b,c):
        if a + b + c == n:
            return 1
        ans = 0
        if num[a + b + c] == a:
            ans += dfs(a + 1,b,c)
            ans %= MOD
        if num[a + b + c] == b:
            ans += dfs(a,b + 1,c)
            ans %= MOD
        if num[a + b + c] == c:
            ans += dfs(a,b,c + 1)
            ans %= MOD
        return ans
    
    print(dfs(0,0,0))

if __name__ =='__main__':
    main()    

Fixed submission:
MOD = 10 ** 9 + 7
INF = 10 ** 10
import sys
sys.setrecursionlimit(100000000)
dy = (-1,0,1,0)
dx = (0,1,0,-1)

ans = 1
def dfs(i,x,y,z,a,n):
    global ans
    if i == n:
        return
    if x == y == z == a[i]:
        ans *= 3
        ans %= MOD
        dfs(i + 1,x + 1,y,z,a,n)
    elif x == y == a[i]:
        ans *= 2
        ans %= MOD
        dfs(i + 1,x + 1,y,z,a,n)
    elif y == z == a[i]:
        ans *= 2
        ans %= MOD
        dfs(i + 1,x,y + 1,z,a,n)
    elif x == a[i]:
        dfs(i + 1,x + 1,y,z,a,n)
    elif y == a[i]:
        dfs(i + 1,x,y + 1,z,a,n)
    elif z == a[i]:
        dfs(i + 1,x,y,z + 1,a,n)
    else:
        ans = 0
        return

def main():
    n = int(input())
    a = list(map(int,input().split()))

    dfs(0,0,0,0,a,n)
    print(ans)

if __name__ =='__main__':
    main()

Buggy python submission (verdict: Time Limit Exceeded):
import sys
INF = 10 ** 15
MOD = 10 ** 9 + 7
sys.setrecursionlimit(100000000)
input = sys.stdin.readline
from math import pi
from functools import lru_cache

def main():
    n = int(input())
    num = list(map(int,input().split()))
    
    @lru_cache(maxsize = None)
    def dfs(i,a,b,c):
        if i == n:
            return 1
        ans = 0
        if num[i] == a:
            ans += dfs(i + 1,a + 1,b,c)
            ans %= MOD
        if num[i] == b:
            ans += dfs(i + 1,a,b + 1,c)
            ans %= MOD
        if num[i] == c:
            ans += dfs(i + 1,a,b,c + 1)
            ans %= MOD
        return ans
    
    print(dfs(0,0,0,0))

if __name__ =='__main__':
    main()   

Fixed submission:
MOD = 10 ** 9 + 7
INF = 10 ** 10
import sys
sys.setrecursionlimit(100000000)
dy = (-1,0,1,0)
dx = (0,1,0,-1)

ans = 1
def dfs(i,x,y,z,a,n):
    global ans
    if i == n:
        return
    if x == y == z == a[i]:
        ans *= 3
        ans %= MOD
        dfs(i + 1,x + 1,y,z,a,n)
    elif x == y == a[i]:
        ans *= 2
        ans %= MOD
        dfs(i + 1,x + 1,y,z,a,n)
    elif y == z == a[i]:
        ans *= 2
        ans %= MOD
        dfs(i + 1,x,y + 1,z,a,n)
    elif x == a[i]:
        dfs(i + 1,x + 1,y,z,a,n)
    elif y == a[i]:
        dfs(i + 1,x,y + 1,z,a,n)
    elif z == a[i]:
        dfs(i + 1,x,y,z + 1,a,n)
    else:
        ans = 0
        return

def main():
    n = int(input())
    a = list(map(int,input().split()))

    dfs(0,0,0,0,a,n)
    print(ans)

if __name__ =='__main__':
    main()

Buggy python submission (verdict: Time Limit Exceeded):
import sys
INF = 10 ** 15
MOD = 10 ** 9 + 7
sys.setrecursionlimit(100000000)
input = sys.stdin.readline
from math import pi
from functools import lru_cache

def main():
    n = int(input())
    num = list(map(int,input().split()))
    
    @lru_cache(maxsize = None)
    def dfs(i,a,b,c):
        if i == n:
            return 1
        ans = 0
        if num[i] == a:
            ans += dfs(i + 1,a + 1,b,c)
            ans %= MOD
        if num[i] == b:
            ans += dfs(i + 1,a,b + 1,c)
            ans %= MOD
        if num[i] == c:
            ans += dfs(i + 1,a,b,c + 1)
            ans %= MOD
        return ans
    
    print(dfs(0,0,0,0))

if __name__ =='__main__':
    main()   

Fixed submission:
MOD = 10 ** 9 + 7
INF = 10 ** 10
import sys
sys.setrecursionlimit(100000000)
dy = (-1,0,1,0)
dx = (0,1,0,-1)

ans = 1
def dfs(i,x,y,z,a,n):
    global ans
    if i == n:
        return
    if x == y == z == a[i]:
        ans *= 3
        ans %= MOD
        dfs(i + 1,x + 1,y,z,a,n)
    elif x == y == a[i]:
        ans *= 2
        ans %= MOD
        dfs(i + 1,x + 1,y,z,a,n)
    elif y == z == a[i]:
        ans *= 2
        ans %= MOD
        dfs(i + 1,x,y + 1,z,a,n)
    elif x == a[i]:
        dfs(i + 1,x + 1,y,z,a,n)
    elif y == a[i]:
        dfs(i + 1,x,y + 1,z,a,n)
    elif z == a[i]:
        dfs(i + 1,x,y,z + 1,a,n)
    else:
        ans = 0
        return

def main():
    n = int(input())
    a = list(map(int,input().split()))

    dfs(0,0,0,0,a,n)
    print(ans)

if __name__ =='__main__':
    main()