I've found this in the wsop tournament rules:
- Exposing Cards and Proper Folding: A participant exposing his or her cards with action pending will incur a penalty, but will not have a dead hand. The penalty will begin at the end of the hand. All participants at the table are entitled to see the exposed card(s). When folding, cards should be pushed forward low to the table, not deliberately exposed or tossed high (“helicoptered”).
So the hand should have continued normaly with P3 making his decision. If the flop was to be dealt, P2 would have to play with cards face up.