P1 is the chip leader.
P1 Button, P2 SB , P3 BB
P1 goes all-in; P2 call; P3 still deciding his move shows his cards.
No other players are active in the hand and P3 was the last one to act. Is P3 showing his hand a legal action?
The question has an answer in this topic:
You need to be careful with this. In some casinos this is perfectly allowed while in others, like most here in Vancouver, your hand is automatically dead the second you show it to another player, regardless of how many people are still in the hand.
Basically, it's up to the casino, but yes - in the situation you described, he could show his cards as a call (again, dependent on the casino).
But generally I suggest only show your cards when you need to: When the dealer announces the showdown.