What do you mean by "full grown?" If they are 2 years old, very obvious which ones are roosters or hens. If they are 3 months old, not as easy. Different breeds have different colors to tell them apart. Some you have to wait until they develop combs, longer feathers etc to determine which. You can't "vent sex" adult chickens, only chicks (like 1 day old).
The three that are not laying may be A) too young to lay, B) stressed by the move (or illness, whatever) and will start again, C) don't lay regularly or D) roosters.
This may sound silly; but, when I started with my chickens in the classroom (from county extension sevice) the lady that came in and worked with my class showed me that a roo's legs hang down and a hen's legs draw up close to her body. I thought that is a weird idea (just had to be a trick or an old wives tale or something) - so I tried it on the 6 babies I was aloud to keep. The leg test showed 4 roos and 2 girls. Well, when they grew up the "test" remained correct. I have done this with each successive year and with the chicks I kept along with 2 that my Little Red Hen hatched and raised - it still worked. To this day, I can still pick up my grown-ups and roos drop their legs down straight and the girls draw them up tight to their bodies!!