Wow, what a story. You daughter was riding a regular bike before three. Is she physically talented in other ways? That seems exceptional!
Me on the other hand, I had a tough time back when I was a kid on training wheels, only really grokking it at 6 or so.
> only really grokking it at 6 or so
Probably the more relevant factor: we replaced one of cars with a cargo bike when she was 15 months old, so she does 1500+ miles a year "on" a bike, and a tiny fraction of that in a car (we live in a totally car-centric US city, so this is pretty out of the norm). Bikes are the fabric of her daily life so she is really, really motivated to spend time on a bike.