Given the nature of this game with jobs starting at level 30 and the need for cross class abilities, I'm going to be different than everyone and say level all three to 35 and see what works best for you in a instance setting (don't be a FATE baby). You're going to want to try them all anyway due to the different styles, 35 isn't that high of a level to get to, and you're going to need the crossclass skills.

That being said,
WHM is the easiest job at lower levels, but the hardest to master at higher levels (most restriction).
SCH is also easy at low levels due to the OP fairy, buy has a different heal style than your normal MMO healer at higher levels (most flexibility).
AST takes a lot of multitasking even at low levels, but the playstyle is flexible and consistent at higher levels, so it's easier to master if you spend time with it (most freedom).