A 3" drain should be fine. The only drawback I found was into the sump I split into 3 and the majority of the water does into there, which played havoc on my sump (made a wave) as it went through the baffles. Fixed it with some acrylic to redirect the water through a baffle
I did vent the line by drilling some holes at the end. I guess I could have T'd it off but wanted to control the amount of flow
Good luck with your build.
I need to update this thread, time just gets away from me