6 plants - 3 are ready so can i . . .

Harvest 3 of them now and leave the other 3 under my 600W HPS for an extra week, will this give the 3 left a bigger boost as they would then have 200W each for an extra week?

Well yeah, but if they're ripe they're ripe and I like 'em super ripe. Receding hairs, giant triangular ball bearing shaped calyxes, orange tinge, dying leafs, no new growth. Smoke the other 3(or wait) and harvest the other 3 before you run out.
Patience for the potent.