250w HPS isn't enough light to properly illuminate a 4x4 foot print. But given your ceiling space you have to flower small plants, so you can crowd them together into the smaller area that will be enough light for them.

+1 on when they hit about a foot tall if your going to try and do all 3 at once.

If it were me given the tent and light, I'd do a single plant in a SCROG. The light should do about 2.5x2.5' Screen nicely, and something like that should yield 2-3oz finished.