Ahri, Confident on Battlefield, enemy attacks with 3 unit. My legends (ahri) Nine-Tailed Fox gives them -1 each. Does Ahri, Confident gets +3 mights?

Yes — if all three attackers are attacking the battlefield Ahri, Confident is at, she gets +3 Might this turn.

Nine-Tailed Fox triggers once per attacking unit (FAQ #12698), so that's three separate Chain items resolving one at a time, and each real −1 is its own "you reduce the Might of an enemy unit here by 1 or more" event for Ahri's second ability — she gets +1 for each, not one lumped bonus (FAQ #12945).

Two things to check: each attacker must have Might 2 or higher, because −1 on a 1-Might unit snapshots to −0 and nothing was reduced, so no bonus for that unit (rule 479.4.b, FAQ #12942). And the reduction must land here — if she's sitting at a different battlefield than the one being attacked, her ability sees nothing.