Dd was this age when stbx moved into a separate room in the house & where we no longer did things as a 'family' except in rare cases & 4.5 yo when he finally moved out. I was as straightforward as possible. I stressed that we both loved her very much but we could not get along anymore. We talked about lots of different kinds of families.
I also set the ground rules with x & his parents that there would be *no* disparaging or negative talk in front of her (I'd say in 2 years, it's happened maybe a handful of times total on either of our parts). Also any conversations with dd were definitive...he was not moving back in, we had to do things this way now, etc. so there was no confusion on her end. I don't believe in involving the child in the particulars, but I definitely wouldn't treat it as a non issue.
I also tried to get more attuned to how she was feeling, what triggered her sadness, anger, etc., not to avoid them but I know she had to process the changes also & as difficult as it was for me, it was 100x more for her since she didn't have the skills to sort out her feelings or even really know what she was feeling or why.
I also understood she just needed more...more attention, more understanding, more love, just more. Dd was still in my bed, & she needed that closeness, which was fine by me. I also made sure any promise I made was kept...to pick her up at preschool, to go someplace, etc. I couldn't control anyone else, but I wanted her to have as much consistency and reliability as possible.
Good luck. It's so hard to know what to do.