Whenever enchanted creature attacks, put a +1/+1 counter on it. Then if it has three or more +1/+1 counters on it, sacrifice Ordeal of Thassa.
When you sacrifice Ordeal of Thassa, draw two cards.
9/15/2013 The +1/+1 counter is put on the creature before blockers are declared and before combat damage is dealt.
9/15/2013 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.
9/15/2013 If you sacrifice the Ordeal in some other way, its last ability will trigger.