What about the cutiemark?

Maybe:

AB's: An apple covered with a flower bloom
Scootaloo's: A scooter
SB's: A music note

I think Apple Bloom's would have something to do with building things, so maybe something like a hammer and nails. Scootaloo would have either a scooter or a flaming wheel, and Sweety Bell something musical, like notes on a staff. Just in my opinion :)

Let's see here...

Apple Bloom will have a cutie mark that looks like a flowering apple seed(Apple... Bloom. How hard can that be to design a cutie mark around that name?).
Scootaloo will most likely get a cutie mark that looks like a scooter, only because she's awesome performing stunts with it(at least, in my mind).
Sweetie Belle will have a microphone for a cutie mark. I wonder if any bands have an opening for a female vocalist. That would be the best way to get that cutie mark, in my eyes. Buck stage fright; when you have your cutie mark, all fears go out the window.

Applebloom: Something that has to do with art
Scootaloo: Something aero dynamic
Sweetiebelle: A microphone

apple bloom:a basket full of apples
sweety belle:music notes
scotaloo:a scooter