I agree with Quan.. You can get by with 2 if you run them high enough but 3 would be best. Especially given the depth of your tank. You would get much better coverage with 3 and have plenty of light up high for the most light demanding corals. In the lower areas of your tank, you would be able to grow less light demanding corals such as softies and LPS as well as some different SPS. The extra light will provide a big improvement overall especially given your tanks depth.