The firstisthe original "Monty Hall" problem (Monty Hall being the emcee of the television game show "Lets Make A Deal"). The second is, of course, the same with the number of doors being the different. The last, I would say, is NOT a "Monty Hall" problem because "Monty Hall" doesn't do anything. The third is just a choice between two things.

However, all three leave the contestant in the same position when he makes the final decision- there are two doors, one of which has the prize behind it. The contestant has no knowledge that would make the prize more likely to be behind one door rather than the other so the two are equally likely to have the prize. The probability, as far as the contestant is concerned, that the prize is behind one specific door is 1/2= .5.