The Free-for-all playlist is the first option that pops up when you go to the option to play online.
Just play three games in that playlist. You need to complete three matches and the practice bouts don't count towards your total.
If you are just logging in, remember to check the daily mission before starting so that you can earn some extra gold on the way
As you don't need to win, or remain as the same character, this is pretty easy