I just went through 6 months of this problem and figured it out last month.

Sounds like the same problem as my cat: anxiety.

From your description, sounds like she’s been tossed into a new environment with two new cats and she’s feeling really stressed about it. Since I’m not a vet or animal psychologist, I can’t really offer that much advice. In my situation, I simply removed the lid from the litterbox because being confined makes my cat even more anxious.

I would test different locations for the litterbox, maybe put it somewhere where the other two cats don’t go or can’t access if possible (such as your room?). My gut feeling, based merely on my own experience, is that she needs her litterbox to be in a safe zone where it only smells like her and where she can’t feel threatened while she’s doing her business.