Here is my reasoning on my current duplicate 3*:
Jahangir… honestly, I probably would’ve only maxed one of them, but I still haven’t landed a 5* Red and I’ve got most of my 4* Red to 3-60 (some at 4-70), so I figured with how fast it would be to level a second Jahangir to 3-50, I might as well. I used BOTH in the 3* Rush Attack (Very Fast) tournament, I put one on each far edge, and I got a Grade B on my defense, so his heavy damage to all enemies seemed to help in that specific case.
Squire Wabbit - I got two of him, and he hits like a truck for single targets, so he is great for 3* rare events, especially vs. green. If I stack them and Namahage, things don’t last long when red tiles hit them.
Muggy - I got about 7 of him, and decided to keep two, since I had the extras there to max skill if they didn’t get there from feeders. He has a really high Attack rating, so it is useful in 3* events against blue… but otherwise I’ve haven’t used him much.
Mnesseus - he hits as hard as Berden, plus dispels all enemy buffs. Great for the 3* Buff Booster tournament, but fairly high attack rating. I also find that heavy damage single-target attacks are the way to go in the 3* events.
Gato & Melia - both have higher than 500 Attack rating, and they both do what they do very well; Gato with heavy single-target damage, Melia with heavy damage to all (plus crit to the whole team, great for 3* Buff Booster tourney).
In general, for me to keep and level a 3* now, it has to have a high Attack rating and hit hard with its special. That’s why a hero like Tyrum isn’t on my list: his Attack maxes out below 400, so even though his special does 310%, it is comparatively weak to someone like Gato with 500+ Attack AND 300% special.