I grow 4 plant with great genetic purple punch Barney farm. 25 liter per one. With scrog, and lst. If i grow under 2*hlg550 rspec=960w is replaced 2000w hps?
What you are asking is for someone to see into the future. Do your best and be happy with whatever you get. If your goal is 3 pounds per harvest, you will get there eventually as more experience you gather.
If your genetics are spot on, and plant health is also spot on you could potentially get 1.5 lbs. It really depends on a variety of factors, but also including how much space you actually cover. You can't force 3 lbs of bud out of only like 10-15 main tops. That would be epic to see, though!
When I run my main room, I have 4 girls in RDWC vegged for about 6 weeks and trellised.
I will typically yield 12-20 oz of flower/plant, so I'd think that a 5-7 gal soil grow should be similar.
Good luck with your grow.