Duplicate 3 Stars

I am at the point where I have 5 three stars built to 266 and two of these have 13 covers. I was wondering what other players do with your duplicates. Do you cash in the 3* covers, keeps the 266 3* and rebuild them (using needed ISO), or sell the 266 and rebuild them?
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Comments
1) how fast I can gain HP
2) who needs iso-8 the most
I gain about 1000 HP every three days, so I could dupe additional 3* without having to sell the champed ones.
However, due to my lacking of iso-8 to champ duped 3*, I will most likely wait until they have reached 100 saved covers or I have excess iso-8 to champ them.
If you need the ISO to build up your other characters, but you've got plenty of HP, then roster them, but don't level them up just yet.
If you're short on ISO and HP, then just level up your higher tiered characters.