For the game to work properly the two lesser prizes" must be of comparable value", and switching doors wins 67% of the time.

If, as has been asserted, there was 1 ea. Megaprize, Goodprize and Clunker, then the game changes.

If your first choice is the GP, you will not be be shown the MP...because the game needs to continue; you will be shown the C. Staying Loses

If your first choice is the C, you will be shown not your choice or the MP because the game needs to continue. So again you should switch. Staying loses.

If you actually pick the MP, you will be shown either GP or C, but you don't know that you actually have chosen correctly. So since you know that in two scenarios out of three switching is correct, you should still switch.

Remember you have no way of knowing where the MP is on the first round, but you maximize your chance of getting the MP whenever you are shown a lesser prize. Using the switching strategy you only lose the MP is picked first (1 of 3); you win if you did not pick it first (2 of 3).

Where the strategy may break down is if you have picked the MP, but are are shown the C. You might stand because you're sure to get at least