solution2.py

class Solution:
    def threeSumMulti(self, A: List[int], target: int) -> int:
        cnt = collections.Counter(A)
        res, MOD =0, 10**9+7
        seen = set()
        for a, b in itertools.combinations(cnt.keys(), 2):
            c = target - a - b
            a, b, c = sorted([a,b,c])
            if (a, b, c) not in seen:
                seen.add((a,b,c))
                if len(set([a, b, c])) == 3:
                    res += cnt[a]*cnt[b]*cnt[c] % MOD
                else:
                    if a==b:
                        res += (cnt[a]*(cnt[a]-1)//2)*cnt[c] % MOD
                    else:#b==c
                        res += (cnt[b]*(cnt[b]-1)//2)*cnt[a] % MOD
        # a==b==c
        if target%3==0:
            a = target//3
            res += (cnt[a]*(cnt[a]-1)*(cnt[a]-2))//6
        return res%MOD