Ordeal of Thassa

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

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.