3я:

#include <iostream>

using namespace std;

int main()
{
  int n, count = 0;
  scanf("%d", &n);
  if (n == 0){
    n++;
  }
  while (n != 0){
      count++;
      n = n / 10;
  }
  if (count == 3){
    printf("Da, tri cifri");
  }
}

4я:

#include <iostream>

using namespace std;

int main()
{
    int n, sum = 0;
    scanf("%d", &n);
    while (n != 0){
        sum += n % 10;
        n = n / 10;
    }
    cout << sum << endl;
}


5я:

#include <iostream>
#include <string>

using namespace std;

int main()
{
    int n, current, last = 0, k = 0;
    cin >> n;
    while (n != 0){
        current = n % 10;
        if (current == last){
            cout << "Da" << endl;
            k = 1;
            break;
        }
        last = current;
        n = n / 10;
    }
    if (k == 0){
        cout << "Net" << endl;
    }
}