that's some sick growth for 3 weeks.....

most important thing for you to consider right now is your grow space. remember during flowering you plant will continue to grow in height and width for the first part of flowering.
if you don't have the light to accomodate the lower portion of your canopy it will essentially be wasted and wont' reach max potential. plants can grow anywhere from 1/3 to 1/2 their veg height during flower.

you can switch to 12/12 anytime. it is best to wait until preflowers show but its not necessary. 4-8 like mikeyjo said is the average time people do it.

but again, i think your most important issues you need to look at is your light amount, and your grow space size....(ventilation too)