If the enemy has a 3 might unit on the battlefield and triggers the Rengar, Pridestalker ability for +1 might , can I respond with gust to return it to hand?

Yes — provided the +1 Might trigger is still on the chain when you play Gust.

Sequence: the enemy plays a unit, the Rengar, Pridestalker trigger goes on the chain (a triggered ability, so it's a Closed State you can respond to), and its target is locked in at finalization. At that moment the unit is still 3 Might, so Gust is a legal play and, resolving newest-first, it bounces the unit to hand before the buff applies. The Rengar trigger then resolves with its target in a non-board zone, so the relationship is severed and the +1 [M] does nothing.

Caveat: this only works while the buff is still pending. If the +1 has already resolved and the unit is at 4 Might, Gust cannot be played at it — the "3 might or less" restriction is checked both at play and at resolution (FAQ #36, #1890).