I would say a male rat is "mature" at 6 mos of age. If you have a separate tank, put one female and the male in there and just leave them alone, they will fight it out. It's usually not too pretty, they will tumble around and be pretty mad about the whole thing. We had to do that with our fancy rats. Just let them duke it out. They got over it pretty quick. With two females and one male, the females are goin to try to protect each other and be more aggressive than if they were by themselves. So I would introduce one female at a time. Be aware though that you may have upwards of 20 babies per each female. Fun stuff