No, Isshtak is not so good, better continue Belith for your green.
3* You’ll probably never use:
Dawa, Renfeld, Prisca, Greymane, Vlad
Others that aren’t great, but may have uses in the future WRC:
Jahangir, Oberon, Friar Tuck, Carver, Arman
Others that aren’t amazing, but I’d keep if you’re lacking enough heroes in that specific color:
Isshtak, Karil
For sure keep:
Red: Rudolph, Squire Wabbit, Namahage, Hawkmoon, Nashgar, Azar
Blue: Gato, Valen, Ulmer, Gunnar
Green: Muggy, Belith, Brienne, Berden, Mnesseus, Hisan
Yellow: Bane, Gan Ju, Melia, Kailani
Purple: Balthazar, Tyrum, Chochin, Gill-Ra
EDIT: @Dragonsgirl hello, you wake an old (6 month) posts, what is your question?