can i use bellow's breath during enemy's turn when i have no units there and they enter the battlefield, it has action not reaction

It depends on how the unit got there.

If the opponent MOVED the unit into that battlefield (Standard Move or an effect): yes. A move into a battlefield opens a Showdown — a Non-Combat Showdown if the battlefield is empty, a Combat Showdown if it becomes contested. Action spells like Bellows Breath are playable during Showdowns on any player's turn while the chain is empty (Rule 806.1.b), so once Focus passes to you (the mover has Focus first), you can play it and kill a 1-Might unit. You don't need your own units there — Bellows Breath just needs "up to three units at the same location," and the moved unit qualifies.

If the opponent PLAYED the unit from hand (or revealed a Hidden unit): no. Playing a unit creates no Showdown, no chain, and no priority window, and outside of Showdowns a spell can only be played on its controller's turn (Rule 155). This is exactly the case where you'd need Reaction — which Bellows Breath doesn't have.

One timing note: you can only play it while the chain is empty. If the opponent opens with a spell first, the state closes and only Reactions can be played.