A single MH will illuminate a 2'x2' area.I think a pair of Mh's will be plenty.400's or 250's,either will work.I only run my 250 for 8hrs and let the actinics fill in the rest of the 12hr photoperiod.I think you'd be better off with just a pair of halides and pair of actinics rather than 3 MH's.250's are more than capable of providing good irradiance at 20'"depth.If your going to have a DSB thats 4"+,then 250's will be fine.However,I know Dakar uses the 400 sakis and loves them.They also produce quite a bit more PAR than 250's.