Welcome to BYC. Glad you decided to join our flock. I wouldn't recommend putting a rooster in with three hens of any age if you care about the health of the hens. The recommended ratio of roosters to hens is 1 rooster for every 10 hens. As they mature and the hormones kick in, too many roosters (or in your case too few hens) will become very hard physically on your hens; over-breeding them, biting and plucking the feathers from their necks and backs, battering them, and potentially, seriously injuring them. The only reason you really need a rooster is to fertilize eggs for hatching and 1 rooster can easily handle 10-15 hens in this regard. Your hens will be much happier without a rooster around and their lay rate will be better (stress from an over-breeding rooster causes a drop in lay rate). Please feel free to ask any questions you may have. We are here to help in any way we can. Good luck with your flock.