How is your tank set up? Plants, rocks, hiding places, etc? And how big is it? Maybe there's not enough space?

My mollies are the most gentle fish in my (29 g) tank. The male sometimes flares at the platies but he's never actually gone after them. Usually all three mollies are busy grazing on algae all day. It might help that my tank has a lot of plants, including a thick forest of hornwort, plus some rock caves.