When to tell my 3 year old?

My daughter just turned three, and while I have primary custody she is the product of a divorce. I don't mind if she knows (although I also get telling her 6+ months I advance is really testing her tiny patience) but more so I don't feel like getting into it with my ex. Not about my remarriage or new baby or anything.

We are pregnant with twins, so super excited - but when did others tell their young but soon to be older siblings they were going to BE older siblings? She's smart as a whip so she might pick up conversation anyway - just curious what others thought.