Does Bellows Breath repeated deal 6 bonus damage if Rabadon's is equipped?

No, Bellows Breath will not deal 6 bonus damage, nor will it deal a total of 6 damage per target.

According to RiftJudge FAQ #8810 and the mechanics of Bonus Damage (Rule 715), here is how the interaction works:

1. Bonus Damage Application: Rabadon's Deathcrown grants 3 Bonus Damage to your spells and abilities that already deal damage. According to Rule 715.1 and Rule 715.2, when an action deals damage to multiple targets, the Bonus Damage is added to each individual damage instance. Therefore, each 1-damage instance from Bellows Breath becomes 4 damage (1 base + 3 bonus).
2. How "Repeat" works: Paying the Repeat cost for Bellows Breath executes the spell's effect a second time. This results in two separate instances of the spell's effect (two separate 4-damage events), not a single combined "6 bonus damage" or "6 damage" event.
3. Total Damage: Because each instance of the spell deals 4 damage to up to three units, if you target the same unit with both the initial cast and the repeated cast, that unit will be dealt 4 damage twice, for a total of 8 damage, not 6.

In summary, Rabadon's Deathcrown increases the damage of each individual 1-damage hit from Bellows Breath from 1 to 4. Each execution of the spell (initial and repeated) will deal 4 damage to its chosen targets.