Its almost impossible to have to much flow in our saltwater tanks. While three 500GPH poweheads puts your total flow at 1500GPH or just over 10 times the capacity of your tank I like to use that as more of a starting point vs a goal.
There are lots of places in your tank where the water flow is almost stagnent im sure due to how the rocks and such affect and direct the water current.
Honestly I cant say you need X powerheads that produce XXXGPH each. Why do I feel I cant difinitvly say this? Because each tank is unique in how water flows in it. How you have your live rock will govern how the water current flows around the tank. I have seen tanks with close to 20 times the tank capacity flow and still have areas in the tank where water flow was almost non existant and then again I have seen tanks that have had 12 time the tank capacity in water flow and evetything in the tank is geting a fairly good water movement.
I think three on the back blowing toward the front is to few though. Maybe two in the corners blowing toward the middle and one or two blowing from the sides along the back and toward the middle. The concept is to produce random water currents in the water.