When do I ask who his biological mom is?

I've been with my boyfriend for 3 months. I've met his parents and his parents are in a poly relationship. He has two mom's and one dad. Obviously I know who his dad but but I just didn't know how to ask who is biologically his mom without making it weird. He clearly loves them all and I feel like me asking would be diminishing the nonbiological parent. We've been together 3 months now. How long should we be together and how should I ask?