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.

Ordeal of Nylea's last ability will trigger when you sacrifice it for any reason, not just when you sacrifice it because of the effect of its second ability.