I'm not the most experienced chicken keeper here so I waited to see if others responded first. I have blended young birds with old before successfully. Your babies appear to be fully feathered so they should be fine but I would keep a close eye on things at first. As long as everyone has enough...
Congratulations on the new additions!! It will take awhile for Penny to come around. And she may not be terribly motivated to get used to them being spoiled in the house as she is!!! Getting them all used to each other likely won't be without "issues" either especially since the other two are...
Rusty's Ranch :
I think we'll set up a quarantine area just to be safe. Even though I only have 1 bird, we kid that she's "Penny the Wonder Chicken" and I'd hate to have anything happen to her. We believe she's not really a chicken, but an alien disguised in a chicken suit. My husband swears...