We had a similar situation, we had three girls and two died from heat in a month leaving our poor lonely EE. We ordered a couple of started pullets from Murray McMurray and they came in about 17 weeks old so similar in size. We slowly introduced keeping them penned separately and after a couple weeks or so turned them out together and watched what happened. The older girl was clearly the boss lady and the younger girls allowed her to stay that way. She seemed fine for a week or two by herself as long as we spent plenty of time with her and then the other chickens came and they were housed in a quarantine pen but could be seen by her. I don't know if this would work in your situation, but it is so sad when that happens! We didn't pen them together for a month just to be sure Princess Laya, our eldest wouldn't go on the warpath, she can be feisty. One thing getting the started pullets addressed for us was size, we wanted girls that could hold their own, but not ones that would gang up on our original girl so the age worked out really well. So sorry this happened, it is pretty sad for the remaining girl and for us too as owners!