3ft plant question

At what height is good to move them to flower if i want my plants to be 3ft at harvest???

That is all dependent on the strain and your lights.For me about 1'

im still pretty noob but from what i found
if you want a 3ft plant
during flower your plant grows 2x-3x its size
so i think 1-1.5ft would give you a 3 ft plant

Grow,Learn,grow

That is a hard question to answer that depends on the strain and its exact genetics. You probably have to tinker around with your existing strain to obtain your own reliable data. All I can tell you that is fact is that if a strain is indica dominant then the growth pattern from vegetative to flowering state is 2-3 times as for sativa dominant strains it can range from 3-4 times its original sature from vegetative to flowering. Hope this helps.