3 weeks is old enugh for a piggie to leave, it depends more on the size of the litter, for example a small litter will reach sexual maturity must faster then a large litter, you know its time to leave when the males start rumble strutting(walking on there toes, purring, and strutting). as for males or females, I would say males. I have found males to be more cuddly and friendlier then females, my males like to cuddle and do no get scared off, by my females try to avoid being cought under all curcumstances, thats just my experiences though. make sure you have a BIG cage, prefrably a C&C cage, dont feed those mixed diets, just pellets, make sure they get lots of veggies, Dark leafy greens in particular. unlimited hay is a must. and warning, piggies can be NOISY. they know there rutiens and they dont let you forget it!