The FAQ for all three sites seems to cover it pretty well.
Super User concerns itself with computer hardware, software, and personal home computing/networking; Unix and Linux concerns itself with *nix operating systems (not specifically a particular distro); and Ask Ubuntu concerns itself with questions in regards to Ubuntu and its official derivatives.
To answer your question more pointedly:
[Ask Ubuntu] is not the right place for:
- Linux Mint, Backtrack, and other Linux distributions (try our friends at Unix & Linux Stack Exchange).
...so you'd do well with Unix and Linux.
