Hi Ironman,
Thank you for your inquiry. There are a lot of different factors that will effect how many and where to place the units.
For example, what type of corals are you planning on keeping? An SPS dominated aquarium will require more flow than an LPS dominated tank.
Will the Seio's be your only source of water flow, or do you have a return pump also providing flow?
For your size aquarium, I would recommend two to three pumps to aid in flow for the tank. If your tank is an SPS dominated aquarium (or will be), I would suggest the M1100 or M1500's for your tank. If your tank is going to be more LPS's and softies, then the M820's would be good choices. You also do not have to go all with the same size powerheads. You can mix up the pumps to create different "strengths" of currents in different areas of the aquarium.
Placement of the powerheads will depend somewhat on placement of corals and rocks. You are generally trying to get good flow over the corals and rocks, while also trying to eliminate any dead spots where detritus would accumulate. Many people will put one powerhead on each side of the tank and position the third to cover any areas the other pumps (and return pumps) are not moving water. This may be on one of the sides or on the back of the tank. You may have to try a few different placements of the powerheads to see what gets your tank the best possible movement for its inhabitants.
Hopefully this will help out with your decision. If you have any further questions do not hesitate to ask.
Keith M.
Marine Depot Customer Service
http://www.marinedepot.com
customercare@marinedepot.com