Ordeal of Purphoros

• WWizards of the Coast2013-09-15
  Official Ruling
  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.
• WWizards of the Coast2013-09-15
  Official Ruling
  The +1/+1 counter is put on the creature before blockers are declared and before combat damage is dealt.
• WWizards of the Coast2013-09-15
  Official Ruling
  If you sacrifice the Ordeal in some other way, its last ability will trigger.

Rulings via Scryfall