If I attack with 6 might and they play a Back Off from Hidden while having 3 might, whose unit would bounce?

Regarding your question on Back Off:

Back Off is a card that stuns a target unit. When a unit is stunned by this effect, it does not contribute its Might to damage in the Combat Damage Step (RiftJudge FAQ #9712, Rule 423.1.b). A stunned unit deals 0 damage, but still requires damage equal to its full, printed Might value to be destroyed (RiftJudge FAQ #2162).

In your scenario, if a 6 Might unit is stunned by Back Off, it deals 0 damage during the Combat Damage Step. If it receives only 3 damage, it does not meet the threshold to be destroyed, remains on the field, and is recalled to its controller's base during the Combat Cleanup phase.

General Combat Reminders:

• Combat Damage Step: Combat damage is not calculated by 1v1 unit-to-unit duels. Instead, players sum the Might of all their non-stunned units at that battlefield and assign that total pool of damage to the opponent's units (Rule 460.2).
• Recall: If after the Combat Damage Step units from only one player remain, that player wins the showdown. If both or neither remain, there is no winner. Attacking units are recalled to their controller's base during the Combat Cleanup phase (Showdown Flow).