In my flower room I have three, 400watt hps lamps. If I am flowering with 3 plants per light I'll get 2.5ounces per plant, dry. When I cut back to only having 2 plants per light, I get a bit over 3 ounces per plant dry. I usually veg for 5 weeks and flower for 6.
It's actually better to have fewer plants and get the small increase in per plant weight than to try and cram 3 under each lamp.