I think you're being a little crazy. I haven't worked with 3 year olds (I taught gymnastics classes with kids as young as 6), but I don't think they are going to be ready for anything structured like a workout.
What you can do:
1) Play physical games: Throwing and catching games, maybe build a little obstacle course with stuff in the gym. Maybe tug or war? Gotta be a million games you could play.
2) Have toys and light equipment so he can mimic you if he wants. Maybe get a plastic toy barbell from WOD Shop (http://www.wodshop.com/wod-toys-barbell-mini/
WFS) or light equipment he could copy you with (for instance, if you did Atlas Stones, you could have a light medball and short platform for him to try it). Could also incorporate more movements he could copy, like burpees. But don't push it. Whatever you do with him needs to be fun.