What did you plan on doing with the three units you have? how did you plan on laying them out?
How far off the water line, determines your coverage. In addition the intensity of the units will also determine how much light reaches the bottom. Each unit will cover a 24" x 24" square, with some light fall off on the edges. if you have a 36" x 36" tank and you place the unit in the middle , then you will have 6" all around where the light will not be as intense but you will still have light. This area can be prime real estate for corals with less light requirements. Using this example you can use two units with 6" front and back of fall off light. this would only leave the front to contend with since the rear has the overflows. Hope this helps.
4 is the number but you could go with 3 and put the rest of your money
into a lightrail3 I have this system on my 325 and I have the lights moving from front to back it has a cool effect like clouds moving over a reef