I have 3 female rats, all a little over 1 year old.
If i we're to purchase a male rat, how would i go about breeding, and would it be successful due to the age of the females?

Also, most humane way of pre-killing the litters?

By a year old rats have a reduced fertility, I have sucessfully bred from a few females over that age but they've had very small litters and have been bred from before, if yours havent been bred from before i really wouldnt advise it as it will probably just over exert their hearts, you would have more success with some younger rats,
You will need to have enough cages to be able to keep pregnant does (female rats) seperate and of course a seperate cage for the bucks (male rats) as he could possibly kill any babies but of course no animal is the same and im sure there are lots of people who find their rats doting fathers!, the does can often fight over the babies and end up killing them too if not seperated. the does will need to be about 3 months old before being bred. which should be about a month after buying them from a pet shop as they sell them usually at around 8 weeks old, over this month watch your does carefully and every 5-6 days youll notice theyll come into 'season' - the symptoms of this being arching back when you touch her back near her tail and sometimes waggling the ears as well which is known as lordosis. Sometimes other does will try to jump on her during this time too, this is the time to introduce the buck, you can either leave the male with the female for a week or so then remove him back to his cage or you can wait for the doe to come in season and leave the doe and buck together for a 'one night stand' then return him to his cage, this last method is better if your keeping more than one buck together in a cage as they'll fight if kept apart for a long time then reintroduced smelling like a female!!

the gestation period is 21-24 days anything over 24 days and there might be something wrong so keep a close eye on any pregnant does.

hope this helps