What's the difference between [Super User][1], [Unix & Linux][2], and [Ask Ubuntu][3]?

In general, what's a rule for deciding where to post a question when I have one (which might apply to any of those three)?  If I have a problem with [Linux Mint][4], which seems likely to be the same in Ubuntu, is it appropriate to post it on Ask Ubuntu?

  [1]: http://superuser.com
  [2]: https://unix.stackexchange.com/
  [3]: https://askubuntu.com/
  [4]: https://en.wikipedia.org/wiki/Linux_Mint