Nordri is extremely good in the long-run, for anyone who lacks a 4* or 5* Blue EDD (Elemental defense down)
that is a fairly rare skill, and will greatly help you in game modes that rely on tile damage - think Titans, Challenge Events.
Long run - Chochin, not so much , even versus other 3*, Treevil does more. and there are many better mana controllers in 4* or 5*, even in Purple - Proteus, or Merlin (who can be trained from one of the Hero Academy levels)
Bjorn is a good hitter in the 3* arena, and you have his costume too, so a fairly good one to keep
I would also finish Ulmer, as the normal defense down stacks well with Nordri, for greater damage. you also have Ulmer’s costume. if you get c-Gunnar, you can do a C-Ulmer (attack up)/ c-Gunnar (defense down and shared damage) / Nordri (Blue EDD) combo for great damage.
Depend on what color you need. Purple or blue they both great in their own way. Nordi and Bjorn is awesome. Chochin is pretty bad and underwhelming. The guy above give great tips for your 3 star team setup.