Gilthud said:

Is it alright to just move on through the campaign even if I don't 3-star them all? I'm just curious and my "3-star OCD" is just nagging at me. Thanks!

Yes!  It's ok.  

First, just get all the way through campaing.  then on hard, then on brutal.  Then come back and 3* them..  At some point you need to 3* all of them to get all the rewards.  You need to be able to solo 12.3 on brutal to farm food.

My experience?  I put a lot into relic keeper who couldn't quite get through brutal.  I went back to Kael and upgraded hime to 6/60 and he can solo any are of campaign on brutal.