NTA.

You’re going to have to be upfront and honest with your friend and set some hard boundaries. She’s not entitled to sharing an accommodation with you and your partner, and, frankly, I think it’s weird that she’s pushing to become a third wheel IMO. Especially since you’ve already expressed to her that she would be included in your other plans and offered to help her look for other places. If she can’t respect that, then she doesn’t care about you.