Here's what ya do.......... get a t-5 4 foot setup in a seperate, smaller space. I say this because it's cheap, cool, and will support the vegging for at least a pound worth of bud, plus you keep a mother or two and do your clones in there also.
Now your answer to the 3 x 400 watt hps will yield a pound? That is entirely up to you and your care/attention to your plants through the grow cycle. Is it feasable? well certainly, you can get alot more than a pound of these lights potentially. A novice low knowledge grower,just following advice and having a slight grip on what he's doing, can average .5 of a gram of dry product per watt of lighting used. A good grower, advanced, highly seasoned/knowledged of cannabis horticulture can get up to an over a gram of dry product per watt
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So again it's up to you and your master plan.
Since you have stated you want this particular yield each and evry month you will have to play a game with timing. Basing this on a roughly 8 week flowering period, you will want to cut clones from your said mother plant or order clones within 4weeks of eachother. Ideally you want to have clones fully vegd 2 your height specifications then add another set of equal amount of clones to your vegging space as the fully vegged ones go to the flowering room.
Your flower room (12/12 ) much bigger space, will consist of two stages of plants each 4 weeks apart, thus, giving you a harvest every month of an easily obtainable,with 1200 watt amount of " 1 pound " You can start with as many plants on your two alternating age groups of cannabis. I would guestimate starting with perhaps 8-12 plants per batch, 16-24 plants total in your flowering room.I have 8 plants under a thousand watt light, for example. If you fully understand what I am going on about here and it makes sense, you will see the rotation
neccessary to achieve this goal you have set. Good luck ~ BCbuddy