Can Bellows Breath target the same unit for all 3 points of damage?

No, Bellows Breath cannot target the same unit multiple times with a single cast to deal multiple damage to it.

According to the official ruling for Bellows Breath:

• The spell deals 1 damage to up to three different units at the same location.
• Each target must be a distinct unit.
• You cannot select the same unit more than once within a single instance of the spell.

Nuance regarding Repeat: While a single instance of the spell cannot target the same unit multiple times, if you pay the Repeat cost, you execute the effect a second time. You can target the same unit with the second instance of the effect, but the damage is dealt in two separate 1-damage instances, not as a single 3-damage hit. Furthermore, because damage is only checked for lethality during the cleanup phase after the entire spell has finished resolving, the unit will not be defeated until both instances have resolved.