strategy to introduce new birds

Hi
I have just bought 3 new hens and am wanting advice on introducing them to the existing 3 that I have. They are sleeping together ok at night but during the day, the 3 older birds are constantly chasing the younger ones. I have had them separated for 2 days - the older birds are fastened in their run and the younger ones are in a fenced area outside.
Today, I have let one of the older birds out with the younger ones- much to the annoyance of the other 2 older ones.
Is there any reason why introducing one bird at a time is a bad idea? The older one still has a go at the others but on a much smaller scale.
thanks
Phil

I introduced 1 new bird to 3 a couple of months ago, initially there was a lot of bullying, but as they were all out in the garden there was plenty of space to give her room, they were fine so long as she kept her distance. At night they all went into the coup together, eventually they have accepted her into their harem. It took several weeks though.

Hi there I have just introduced 6 new birds to the 6 I originally had on Saturday, I had my original 6 about 5 weeks I put them all in together on saturday morning there was feathers flying everywhere at first Saturday night the all went into the coop Sunday morning there was the odd squobble but now its a case of the odd peck at each other .So i would put them all in together but keep a close eye on them.