As others have already told you, you are not alone in this dilema. Your acknowledgement of needing healing for yourself is key to being able to have answers for your child.
Simplicity is the goal when answering such questions from your 3 year old. In reading your post, it seems you may need to sift through your own questions, and be carefully attentive to her question.
In my world, divorce, separations, remarriage and even death is an unfortunate reality in marital realtionships. But for our family we remain focused upon our love for the individuals despite their actions. What becomes important is establishing for our children that the relationship they have with grandpa never changes no matter how many times he remarries (my dad is divoreced 3 times, and his current wife battling an aggresive cancer). Meanwhile, mom's bitterness from the divorce has startling mainfestions as her alzheimer's progresses.
It is tragically sad that our children have to face such things, but I am grateful that they have the stable model, as yours do, of my in-laws and my husband & I. So we keep the focus on the actual impact these things have on them; i.e., abuelita (my mom) is confused, grandpa (my dad) will always be your grandpa, etc.
The most important part is to make peace with it for yourself, otherwise, your anxieties will be what is passed on to your children.
ABOUT ME: Adult Child of divorce, married 11 years to adult child of healthily married parents. 3 children ages; 7, 5 and 2.5.