Hello,
I just went through a similar situation. My two original hens (silver laced and colombian wyandottes) were about 20weeks and my new hen that I was trying to integrate was about 14 weeks. The first time they met, the two original hens wanted to attack the FBCM (French Black Copper Marans) ferociously. I had to segregate the new hen inside the run within her own cage. I would let the two older hens free range most of the day and keep the FBCM free to explore the run/coop. I did this for about a week. At the end of the week I started placing the new hen into the coop while the other two were sleeping. They tend not to attack in the dark. In the morning I would segregate them again with the free ranging/coop tactic. Although I made sure to get up early so that they did not start attacking her when the sun came up, I found that although there seemed to be stress in the enclosed coop, the hens were not physically fighting. This went on for a few days at which point I stopped segregating them as the older hens no longer seemed that interested in the new hen. They would still assert their dominance/pecking order, but it was no longer as harsh. Fast forward a couple more weeks and now they roam around the yard together but the younger one is still a bit cautious around the older two. It takes time, and you have to understand there will be some abuse by the dominant hens, but it will wane over time.
Good luck to you.
Tony