Ordeal of Purphoros

Enchant creature
Whenever enchanted creature attacks, put a +1/+1 counter on it. Then if it has three or more +1/+1 counters on it, sacrifice this Aura.
When you sacrifice this Aura, it deals 3 damage to any target.

Pioneer Masters

Jumpstart 2022

Enchant creature
Whenever enchanted creature attacks, put a +1/+1 counter on it. Then if it has three or more +1/+1 counters on it, sacrifice Ordeal of Purphoros.
When you sacrifice Ordeal of Purphoros, it deals 3 damage to any target.

Theros

Enchant creature
Whenever enchanted creature attacks, put a +1/+1 counter on it. Then if it has three or more +1/+1 counters on it, sacrifice Ordeal of Purphoros.
When you sacrifice Ordeal of Purphoros, it deals 3 damage to target creature or player.

Duel Decks: Heroes vs. Monsters

If you sacrifice the Ordeal in some other way, its last ability will trigger.

The +1/+1 counter is put on the creature before blockers are declared and before combat damage is dealt.

The check of whether the enchanted creature has three or more +1/+1 counters on it happens as part of the resolution of the attack triggered ability. If the third +1/+1 counter is put on the enchanted creature any other way, you won't sacrifice the Ordeal until the next time the creature attacks.