kits shouldnt rly be out for play time. They need to conserve there energy to growth. They also have issues regulating temperature at that age. They could easily over heat and cause problems. They also often dont know when to stop and over exert themselves.
Just to say again, cage bars need to be at least 1/2" spacing or less. Even a chin a few months old can still get through, or try to get through and get stuck, on bars that are 1" spacing.
As for putting all three together, i would first, naturally, double check and make sure they are all females. After that, i would probably wait until the kit is done nursing at 8 weeks and has been fully weened and eating/drinking on its own. If anything should happen, its (slightly) better if the kit is able to take care of itself a bit.
As for the cage itself, i most definitively wouldnt expand it until after weening is successful. A mother with more room could get away from the kit and possible start to neglect it. You want to make sure momma cant get away from kit like that. So id wait until fully weened.
After that you need to be careful while expanding the cage. Young kits are still clumsy, and have sight issues/ issues judging distance. As such you need to design the cage so that should the chin fall from any spot, that it is a short one.