AB: something with construction, im not sure if it should have an apple shaped something. Idk if that matters, or what.
Scoots: a wheel with fire coming out(same as Wildfire's)
SB: a music note/heart
But you see, i don't think it should be blunt like that, i think Scootaloo's cutie mark wouldn't be a scooter, bc thats not her only talent, and although Sweetie can sing very well, i don't think it would just be a mic. But you can think that, this was just my opinion.