{Ordeal of Heliod}
The wording "then, if it has three or more +1/+1..." Kind of made me think - does the aura only check the condition when the enchanted creature attacks?
I believe this is true
No. The sacrific the enchantment effect is a static based action. Every time counters are put on the creature, the enchantment checks itself for 3 or more. If it is 3 or more, it sacrifices
It only checks when you attack. Just like how {maze's end} only checks when you use it's ability.