I have one on my 75 and I could only have one more on it if I wanted to. Even without the dividing thing on mine that's in the way I don't think I could fit a third.
I dont think I would be able to fit three on there if I cant fit 4 because of the divider Ill have to see ill go to the store and measure it im selling off my 37 gal tommorow I think and then I will have enough money and the 75 gal will be mine.
You could probably add an AC70 to each side. They are about 8.5" each and 300 gph. Or just upgrade to two AC110s 500gph(13.5") and two AC70s. Give ya 1600 gph.