The *ideal* flow rate on your sand filter is 12 - 15 GPM/sft or about 48 GPM. Higher rates reduce filter effectiveness, even though all the manufacturers rate their filters at 20 or even 25 GPM/sft.

The (3) 1.5" returns don't limit you, but if you only have 1 suction return, that does limit you.

I'd recommend sizing for the lower of your filter flow (48 GPM) or your suction piping.