When an ability is triggered on one of your opponent's units without Backfire or with Backfire 2 or less, that unit gets Backfire 3 and "Whenever this unit's Backfire is triggered, put 1 damage counter on it." until it is discarded.
Keywords
Backfire
Backfire X: A cumulative ability triggered by the initial dice roll that means, "When you roll at least one natural three with this unit's attack dice before re-roll, this unit does X dice of damage to itself when the attack ends." Backfire applies only to dice rolled for an attack, not to dice rolls for abilities like Retaliate.