hi there @FreeQuincyWAves , welcome to the game and to the forum!
I think it’s good to be cautious with your heroes, save one copy of each. You never know when you will get them again. (except for the 3* and 4* that come from training camp, they come fairly often… but even then it takes time, it took me 14 months of playing to get my first Rigard for example).
but indeed, the experience boost you get from feeding a 3* is not so big, and it may be more valuable to keep them. depending on how new you are, 3* will be a key part of your hero roster for a while
for Mok-Arrr, what ascension level is he? some notes, the costume cannot be higher ascension than the hero (if the hero is ascension 2 level 45, denoted by 2/45, the costume cannot be ascended to 3rd ascension). the costume can be higher level than the hero for same ascension, so you could save time there (e.g., if Mok-Arr is 4/1, you can keep him there and bring costume to 4/80). you will get some annoying notifications while levelling the costume up though 
if you like, you can put your roster here or in the FTP rebellion thread for individual advice on who to level and who to keep
for costumes, here is the FAQ, and I quote the relevant part below