I'm pretty sure if you have a canopy on a 75 gallon, you can't fit 3 350's.
Prices must be different in the states.
Since 350's are 45 each and 400's are 60 each
at bigals
It would be 3 350's (costing more) or 2 400's
I would pick the two 400's with no question (since thats what I did)
Anyway, people pick different filters for different reasons.
Odds are if you really need 3 350's on a 75 gallon (which is the tank we are talking about here, someone wants to know what they should get for their 75 gallon) then you may want to change your stocking since that is more than enough filtration for a 75 gallon.