The head loss calculator and overflow size calculator on RC are really valuable for figuring this sort of thing out rather than by trial and error.
http://reefcentral.com/index.php/head-loss-calculator
http://reefcentral.com/index.php/dra...flow-size-calc
The overflow size calculator recommends a 2-1/4" hole for 3000gph.
My own tank runs a Dart from which I get about 2300gph after head loss. My overflow is 2" but I have it dialled back slightly so that it sucks less air and makes less noise. I used to have a 1-1/2" bulkhead and when I did I had to dial back the pump because the drain couldn't keep up.