Initializing 3D Vectors using C++

#include <bits/stdc++.h>

using namespace std;

int main() {
    int n;
    cin >> n;
    vector<vector<int>> a(n, vector<int>(n));
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < n; j++) {
            cin >> a[i][j];
        }
    }
    vector<vector<vector<int>>> collect;
    collect.emplace_back(a);
    for (int i = 0; i < (int) collect.size(); i++) {
        for (int j = 0; j < (int) collect[i].size(); j++) {
            for (int k = 0; k < (int) collect[i][j].size(); k++) {
                cout << collect[i][j][k] << " ";
            }
            cout << '\n';
        }
        cout << '\n';
    }
    cout << '\n';
    return 0;
}