I actually got the same one for my 240, but with 4 x 250 MH. If your 240 is the same length as mine (7 feet), then the 3 x 250 MH will leave about 2 feet of unlit space on either side of the tank. Go with the 4 x 250 MH, that at least gives you 1000 watts plus, vs. just 750, which is not much at all for a 240.
This is the one I have off of Marine Depot, but you can find it cheaper elsewhere. I can't remember which website I got it off of, but it was significantly less.