In bridge, is there a penalty if dummy lays down his hand and places a spade with the clubs and it is not noticed by the players until the third round is played?
As a general guideline, all players at the table are jointly responsible for the play and management of Dummy's cards. Consequently, in accordance with Law 64 of The Laws of Contract Bridge (2014):
Law 64 - Procedure after Establishment of a Revoke
D. there is no penalty for the established revoke ...
3. if the revoke was made in failing to play any card faced on, or belonging to a hand faced on, the table, including a card from the dummy's hand, or ....
So although a revoke in Spades by Dummy may have occurred and become established, there is no penalty except as provided in the recap of Law 64:
When any established revoke, including one not subject to penalty, causes damage to the non-offending side insufficiently compensated by the Law, the offending side should transfer additional tricks so as to restore equity.
If no revoke occurred before the discovery of the Spade amongst Dummy's Clubs, then there is no basis for a penalty. However if one of your opponent's is competent at counting to 13, and you are playing for stakes above a couple of cents per point, then I would keep a sharp eye on said opponent in future contests. I have never known a strong player (meaning one who habitually counts as part of playing the game) to not immediately notice any error in Dummy's hand.
As an aside: I am sure that every duplicate club has had a scenario like the following occur every few years:
Pass - Pass - Pass - DIRECTOR!
"This hand has not been shuffled since last week, when I made 8.5 match points with my Right-Hand Opponent's cards for 1NT just in."
That is how well strong players remember all 52 cards of a hand, even long after it is over and scored.