Adding chicks in with the old

This seems to be a good method of introducing new chicks https://www.backyardchickens.com/a/introducing-new-chickens-using-the-see-but-don-t-touch-method
Best wishes!

 

What does "is plenty big enough" mean... in feet by feet?
You will need 'extra' space separate but adjacent for integration.

Single bird integration, especially into a tiny 'flock' can be tough.
Here's a coupe threads that might help:
https://www.backyardchickens.com/a/reintegrating-a-recovered-hen-to-a-small-flock
https://www.backyardchickens.com/t/1074403/success-story-integrating-a-single-hen-into-a-flock

 

Place the new bird in a wire dog crate with food and water for about a week inside the coop. look but no touch...After a week try letting her out..It will be a bit mean looking to you...A lots of chasing and pecking might happen. As long as no blood is she do not interfere..Pecking order needs to be established before they will get along..It could take weeks before the other two allow her into the mini flock you have...Adding two birds would be better because the two you already have will gang up on the single new bird. Also have two feed and water stations going...

Best of luck..

Cheers!