a closet grow with 3 plants.... and you want to run a 400w hps?
your gonna need one helluva inline fan and some ducting to keep it cool.
i would suggest a 150w CMH (ceramic metal halide) and maybe supplement that with (2) 43w con agro cfl's
400w for a closet is some serious overkill in my opinion for just 3 plants... even on a scrog. its just too hot.