虽然知道要用深搜，但我依然选择了先暴力一波

#include<bits/stdc++.h>
using namespace std;
const int N = 10000000;
int sum[N],su = 1;
int n,s[21],last;
int main(){
	scanf("%d",&n);
	for(int i = 1; i <= n; i++) scanf("%d",&s[i]);
	for(int i = 1; i <= n; i++)
		for(int j = i + 1; j <= n; j++)
			for(int k = j + 1; k <= n; k++){
				sum[su] = s[i] + s[j] + s[k];
				if(i = n - 2) last = su;
				su++;
			}
	int ans = 0;
	for(int i = 1; i <= last; i++){
		for(int j = 1; j <= sqrt(sum[i]); j++){
			if(sum[i] % j == 0){
			sum[i] = 0;
			break;
			}
		}
		if(sum[i] != 0) ans++;
	}
	printf("%d",ans);
	return 0;
}