a single 1 1/2" Drain should be good on the 125. drains aprox 1200gph. what kind of return pump are you going to be using. the pump will determine the amount of water that will drain.
For 2400 gal of flow and return I agree with your initial though of one on each end and 1 in the center. This will give you definitely plenty to do as you like for returns. With this kind of turnover you should not need any pumps inside the tank but you will have to make sure your returns to the sump do not restrict flow down.