Bendu had 3 HP remaining, so the result of 3 (highest among 1, 1, and 3 as all are rolled together and take the highest to "gang up" on him) applies to him. 3 HP with 3 damage = 0 HP, so we roll on the impact table given the blow. The "he's safe" aspect refers to him absorbing all incoming damage to his HP, as anything beyond that would go to his STR score, which would trigger a critical damage save. Failing that, he'd be down for the count until rescued.
In short, any time your HP is reduced to exactly 0 from a damaging attack, you roll for the impact.