Duty Rota please!!

Hi everyone, I would like to ask you guys please if anyone of you have a program that can help me to make the Duty Rota

My problem that I have 28 nurses
There are 3 shifts during the day

The first shift from 7:00 A.M to 2:00 P.M
The second shift from 2:00 P.M to 9:00 P.M
The third shift from 9:00 P.M to 7:00 A.M

In the first shift the ward should have 13 nurses. However, in the second and the third shifts the ward should have 3 nurses. Added to that, during the weekends the ward should have 3 nurses in all shifts

I hope someone can help me. It seems to me a mathematics not nursing