3" pipe is 3½" OD. The OD of any fitting sockets, couplings, etc. is nearly 4",so that does not work in a 2X4 wall. Any wet wall is easier to work with if you make it 2X6. Trying to mount a shower valve for proper reveal, in a 2x4 wall, can be tough, but it is doable. It was the norm in a lot of construction. I have seen house where they put the drains in 2x4 walls, and just let the drywall bow out where necessary. Not pretty.