10 gauge whip off disconnect box fed from 12/3 on 15 amp breaker for Mini Split?
As long as the terminals on your mini-split accept 10AWG, you're fine
I'd go ahead and use the 10AWG whip as long as your mini-split's terminals can accept 10AWG wire (most power connections can).  If they can't, then I'd get some black and green 12AWG THHN  to replace the 10AWG the whip came with.