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
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