#include <iostream>
#include <stack>

using namespace std;
int main()
{
    unsigned short n1, n2, n3;
    int a, n=0;
    stack<int> group1;
    cin >> n1;
    for (int i = 0; i <n1; i++) {
        scanf_s("%d", &a);  group1.push(a);
    }

    stack<int> group2;
    cin >> n2;
    for (int i = 0; i < n2; i++) {
        scanf_s("%d", &a);  group2.push(a);
    }

    stack<int> group3;
    cin >> n3;
    for (int i = 0; i < n3; i++) {
        scanf_s("%d", &a);  group3.push(a);
    }

    while (!group1.empty()) {
        if (group1.top() == group2.top()) {
            if (group1.top() == group3.top()) {
                n++;
                group1.pop();
                group2.pop();
                group3.pop();
                if ((group1.empty()) || group2.empty() || group3.empty()) break;
            }
            else if (group1.top() > group3.top()) {
                group1.pop();
                group2.pop();
                if (group1.empty() || group2.empty()) break;
            }
            else group3.pop();
        }
        if (group1.top() > group2.top())group1.pop();
        if (group1.empty()) break;
        if (group1.top() < group2.top())group2.pop();
        if (group2.empty()) break;
    }
    cout << n;

}

what's wrong?

after else group3.pop();
 if (group3.empty()) break;
accapte