Guinea pigs are great pets!

3 weeks old isn't too young. Guinea pigs are able to walk the day they are born, and reach sexual maturity at 5 weeks, maybe 4, so the males have to be away from their mom before then.

I've heard that both make good pets though, but I've only had females. There isn't really much of a personality difference between males and females, except that males will fight with other males if not introduced as babies. So if you want two males you have to get them at the same time, both as babies.

Good luck! ^_^