I started out with 8 multies in a 10 gal. with 10 shells. They paired off. I now have about 25. The fry are very small and hide in the shells for a while. One of the males makes the rounds with a few of the females that don't have a mate. I have found that the males are usually a bit larger than the females.
Just leave them alone and they'll do the rest. I would agree that they are a 3.