I've done a lil reading, and I came to the conclusion that:
1 250w hps will give out in the region of 26,750 lumens.
Divide by 3 plants, and they'll be getting just under 9000 lumens each... which I think is pretty good. Though I guess you've gotta take into account the fact that the lights will have to be distanced from the plants, thus losing intensity from the plants perspective.
1 400w hps will give out in the region of 42,800 lumens.
4 plants @ 10,700 lumens each, or 5 at 8560 lumens.
I'm thinking that a 250w would probably do nicely for growing 3 plants with pretty nice tight buds.
Anyone wanna share their thoughts ... ?