Just using simple math the 1.25 provides almost the same area as three 0.75 hoses, more than three 0.625 hoses. The 1.25 will flow plenty, 1" might be enough. I would not use a scoop, use a simple thru hull so they will fill and empty at the same rate.
I sold my boat with the Simer pumps, my next system will be aerator based to cut down the noise. After having spent a summer with the Simers I think aerators work better for ballast pumps, they just take a little more engineering to get them to work correctly.