How old are the girls? Are they laying eggs yet? Usually one of the easiest integrations is to introduce a mature rooster to mature hens. He wows! them with his magnificence, mates one or two, and it's over. There may be a bit of chasing and running away but usually that's more like flirting than anything else. With living animals you don't get guarantees, anything can happen, but a lot of the time it is this easy.
At 10 months he is probably mature enough to act like a mature rooster instead of an immature cockerel. If the girls are laying they should act like mature hens. I would try putting them together and see how it goes. In any integration you need to be prepared for anything but I would not hesitate to try.
If the girls are immature pullets and not laying yet I'd suggest housing them across wire from him for a week or so, then see what happens. Base what you do after that on what you see. Again, it is often this easy. But sometimes it's not.
I would not lock all three in the coop together to start with. Give them as much room as you can when they integrate. Room is very important in integration. He is probably already sleeping in the coop. If they are mature he will probably lead them to the roosts at night but always check to see what actually happens. If the girls are immature and you are going to separate them build your pen to house them across wire in the coop if it is big enough. That should train them that the coop is home.