Which 3* mystic should I rank up?

Chainslayer

Cassy

None, rank 5*

Chainslayer

I need one for arena grinding. All my 3 stars are rank 1 only.

Cassy

Then what you like playing
Or Look for Synergys that help. (trinitys!)

But beyond the small Arenas there is less Use for them.
Until youre sitting on a huge Gold Pool, leave them alone and go for 4* for Arena. Dropping even more points...