C++ :

#include<iostream>
using namespace std;
int main(){
	int a[100001],n;
	int b=0;
	int c=0;
	cin>>n;
	for(int i=1;i<=n;i++){
		cin>>a[i];
		c++;
	}
	for(int i=c/3+1;i<=c/3*2;i++){
		if(a[i]==3){
			b++;
		}
	}
	cout<<b;
	return 0;
}

Python :

# coding=utf-8
a=int(input())
n=input().split()
sum=0
start=int(a/3)
end=int(a/3*2)
for i in range(start,end) :
    if(int(n[i])==3) :
        sum+=1
print(sum)