Ordeal of Heliod — ruling 1

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

Ordeal of Heliod — ruling 2

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.

Ordeal of Heliod — ruling 3

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

Ordeal of Heliod : règles, fonctionnement et interactions (MTG)

Ordeal of Heliod is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

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, you gain 10 life.

How it works : Ordeal of Heliod

Original English text — Wizards of the Coast / Scryfall

#1September 15, 2013· Wizards of the Coast
The +1/+1 counter is put on the creature before blockers are declared and before combat damage is dealt.
No other card is named in this ruling.
#2September 15, 2013· Wizards of the Coast
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.
No other card is named in this ruling.
#3September 15, 2013· Wizards of the Coast
If you sacrifice the Ordeal in some other way, its last ability will trigger.
No other card is named in this ruling.