You can actually keep 3 but they would need to be all male or female as they breed otherwise and you don’t have room for that. That is with no other fish also. Be sure your tank is cycled before adding them. Read up on the nitrogen cycle for more info. Good luck!
Personally I would not keep any adult platies in a 10 gallon, unless it was unusually long and shallow. Platies need at least 24 inches swimming room. It would be OK as a grow out tank for baby platies but that's all.