How can I keep young kittens from suckling on each other's genitals?

I guess it is just baby animal week for me.

My girlfriend found 3 small kittens on the side of her grandmother’s house. We left them there, in case mom came back. A few days later, mom still didn’t come back, so she took them home.

The vet says that they’re ~3 weeks old, so we can start weaning them this week. The problem is, they keep suckling on each other’s genitals. How can I keep them from doing that? Kitten pants?