2020-11-10 : If you sacrifice Ordeal of Nylea in some other way, its last ability will trigger.

2020-11-10 : The +1/+1 counter is put on the creature before blockers are declared and before combat damage is dealt.

2020-11-10 : 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 Ordeal of Nylea until the next time the creature attacks.