Abuff

I am building a new home theater. Room is appox 18 x 16. I am planning to have RP280F fronts, RP450C Center, 2 RP250S Surrounds and 2 CDT-5800 for Atmos. Planning for PB-2000 Sub.This will be a 5.1.2 set up. The room is a fully enclosed room, with just a door for entry. My question relates to the placement of the 250S. Our listening position is roughly 3 feet from the back wall. Should I place the 250S on the backwall or the side walls of the listening position? Please help. Thanks