Just "caught up" with this thread...must have slipped thru the cracks whe I wasn't looking...
The important point here is "What is the total flow going to be going "INTO" the tank ?"...Then size the final drain line to meet that need...
What I'm saying is ...it doesn't matter how many or how big the overflow lines are...if your only pumping "X" gph into the tank...then you only need "Y" sized pipe to get it to the basement...Doesn't matter if you have Four 3" drain lines coming off the tank, you still only need a "Y" sized pipe to take it to the sump...
So what is "X"...? How much are you planning on "pumping into" the tank (not counting supplemental or "in tank" circulation)