Oh, introducing a new chicken to a flock is the hardest part of chicken keeping!! But it's worth it, because after the initial scuffles it will all settle down
Because she is an ex-battery hen and she will be a little delicate at first, and it may be best to give her a week or so entirely away from the other two chickens so she find her feet and work up some stamina. You'll have to see what condition she is in to begin with; if she's almost fully feathered and walking fine, you can begin the introduction after a few days. If she's almost oven ready (very few feathers), is limping, bruised or in any other way injured, I'd wait a week before beginning the introduction process.
Okay, here is how I introduce a new hen into the flock...
Keep the new chicken in a separated area, but so they can all see each other, for a few days - just short of a week. If you can use netting that the chickens' heads will fit through, even better! That way, they will explore each other's territory, and the flock can give the new girl a peck to show who is boss!!
After they have been living beside each other for a while, when the existing flock go to bed and get shut in their sleeping area at night, put your girl in there with them. Next morning when they are let out, put her back in her separate area. Let them start spending time all together, free-ranging perhaps, for an hour or so. There will be fights, only interrupt if it looks like the hen is being seriously hurt. The odd peck is normal, but if one of the hens stands on your girl and starts seriously attacking, then intervene.
Keep putting her into their room at night, and letting them spend time with her in the day. After three nights of this, let her stay with them throughout the day. There will be pecking, there will be fighting, and someone will just need to be there to make sure it doesn't get too serious.
It will be fine, but there will be fighting. This way is quite time consuming, but will be less stress to your girl overall. When you are leaving your girl in with the flock, make sure there are plenty of food and water containers so the other chickens can't 'starve out' the new member of their flock! Also, if at any time your girl is bleeding, she needs to be separated until she is fully healed.
Good luck with the introductions, it'll all settle down and she will be fully integrated in no time