How much damage carries over, or which attack triggers the end of Polymorph?

Question

Polymorph is said to be a great spell, and I can see why: it can turn a CR 18 monstrosity into a CR 0 Frog, or something along those lines. My question comes from one specific paragraph in polymorph's text:

The target assumes the hit points of its new form. When it reverts to its normal form, the creature returns to the number of hit points it had before it transformed. If it reverts as a result of dropping to 0 hit points, any excess damage carries over to its normal form.

Say a fighter, level 11, attacks a creature that has been polymorphed into a Frog (AC of 11, 1 HP). The fighter has 3 attacks thanks to his Extra Attack class feature, so when the fighter uses his action to attack, he attacks the frog three times.

1 hit is guaranteed to exceed the 1 HP of the frog.

Would that attack end the polymorph spell, and the remaining 2 attacks be against the original creature's AC and HP? Or, would the polymorph end at the end of the Attack action, meaning the fighter got all 3 attacks against the AC 11 Frog?

Answer 1

The polymorphed creature would revert after the first attack.

The Polymorph spell text says that the spell ends when the creature reaches 0 hp.

The transformation lasts for the duration, or until the target drops to 0 hit points or dies.

When the opponent takes the Attack action, each attack has its own attack and damage roll. So if the first attack drops the polymorphed creature to 0 hp, the spell ends and they revert forms and take any damage left over. Then the opponent continues with their other attack rolls, which would be done against the AC and HP of the creature's normal form.