J.,
Your dd is just 3 years old, which is extremely young to process something that complex.
My advice would be to keep it very simple without much elaboration. Something like,
"Her family is moving right now. Maybe when she moves to her new house, you can have a play date with her." Her only question is about playing with her friend.
I had to break the news of my mother-in-law's death to my 3 year old sons,
and I've found it's best just to keep it simple. They can't process complex events
at that stage. Expecting her to process a marriage breaking up is unrealistic
and could cause her to have doubts about your marriage. If her mommy and daddy
are breaking up, will my mommy and daddy break up??
I am child of divorce too and still bear the scars from years of parental wars.
You are too, so you know the drill. Keep your explanation simple, answer
her questions with short explanations (not long answers) and let it go. She'll be fine.