If I play Bellows Breath and there is only one legal target on opponent's side, can I deal all three damage to it?

No, you cannot deal all three damage to a single unit with one cast of Bellows Breath.

According to the RiftJudge FAQ #168 and #1871:

• Bellows Breath deals 1 damage to up to three different units at the same location.
• Each target must be a distinct unit. You cannot target the same unit multiple times with a single instance of the spell.

Even if there is only one legal target available, you can only choose that unit once to deal 1 damage to it. The remaining damage instances cannot be applied to that same unit.