... as if everyone didn't already know how to. The official Tales of the World: Radiant Mythology 3 website has been updated with a new system video regarding Job Class changing.
Like before, class changing is done by talking to the leader of the Ad Libitum guild. Some classes will only be unlocked if you have reached a specific level of its prerequisite job. If you'll notice, the list of jobs has 16 slots. With Radiant Mythology 2 already having 15 job classes, and with the new Gunman class having been recently announced, it's safe to assume that we won't be expecting any more new job classes for this game.