To use a transition fitting with 3" pipe you will need this special bushing. Fits in the upper end of transition fitting and can be adjusted to place pipe closer or farther from the wall. Lower end of transition fitting is 4" OD and would require a 3x4 coupling to connect to 3" PVC pipe or a 3" radon fan.

Can also be used without the transition fitting to adjust the position of your PVC pipe.

See the transition fitting here.