I would build a new coop and put the new ones in there. That way there is not so much (way less )territorial behaviour to be expected. If the new coop is better the other two probably join one day.also! would it be better to build a better coop and give them time to get used to it before bringing in new ones?
Keep the old coop. A second coop is always handy if you have a broody or a sick chicken.