Ballet School

Depends on where you live I guess. I just moved to Woluwe area and have a 3 yr old as well. I have been looking at programs at La Woluwe centre sportif). It's in french but you can use google chrome browser to translate or just copy-paste it into google translate)
http://www.centresportifdelawoluwe.be/fr/danse-classique/index.phrp
They also offer another one which combines 3 types of dances for little ones, a special program adopted from the U.S.:
http://www.centresportifdelawoluwe.be/fr/modern-jazz/index.php

Anyway, hope this helps, good luck. And if Woluwe is far from where you live, I am sure you can look at the sports center of your neighborhood.

There are dozens of them. If you stated the area of the commune you live in, I could suggest more.

Also whether you prefer French, Dutch, English or another language.

I would suggest you try and sign up for a trimester first before paying for a full year, having found out the hard way after pulling my 4yr old daughter out of her ballet class mid year because she hated it (she loved it for the first 3 months) but modern ballet requires discipline - something that not many 3 year olds have.
Have you thought about gymnastics?

We're happy at ours - and Marie just started inscriptions for next school year.

http://www.lespetitsratsduchatelain.be

courses held in French.