Ive never seen 150W MH.. ive seen 175's.. but anyways...
If your planing on keeping anyhting on the sandbed, i'd go with the 250's. But, if everyhting is going to be atleast 6" up on rocks or so, the 150/175 should be fine. Also, a good reflector would help you stay at the 150/175's as well. But id agree with you on definitly needing 3.
OT: What other lights are you using? Im settin up a 155 (Same dims as the 180, but a bowfront), and im doing 3 250's. Trying to figgure out my atinics and stuff like that.