#include<iostream>
#include<map>
using namespace std;
int main()
{
    map<int,int>a;
    int n,k,i,m,l,p=0;
    cin>>n;
    for(i=0;i<n;i++)
    {
    cin>>k; a[k]++;
    }

    cin>>m;
    for(i=0;i<m;i++)
    {
    cin>>k; a[k]++;
    }

    cin>>l;
    for(i=0;i<l;i++)
    {
    cin>>k; a[k]++;
    if(a[k]==3){p++;}
    }


    cout<<p;

    //system("pause");
    return 0;}

thanks for sharing your solution.

maybe, a is a very long array.
I understood this method, but I have to worry the MLE...perhaps, "map" is magic...