Bluecat & Dr Adequate have already answered your questions very well.
However, I would add that when you say, "Why cant these simply be varieties of whale...?" - they are! Evolution of species is the change from one variety to another - over time. That is exactly what these examples demonstrate. The great thing about fossils is that they can be dated (by a number of different methods), and you don't find any evidence of whales existing before the first land animals appeared, nor for the next few hundred million years. That is the same with the whole fossil record - it is absolutely consistent in demonstrating changes from one variety to another over time.
The whale fossils are a great example of intermediates because they show how the nostrils/blow-hole moved back from the end of the snout to the top of the head over time. This is exactly what evolutionary theory would expect (or even require) to have happened, and the fossils have now been found. The dating of the fossils fully supports the theory.
While there may only be 3 intermediate specimens in this example, I'm sure there are many more. In any case, as has been stated many times, it is incredibly fortunate that there are as many fossils as there are. It is just not realistic to expect a perfect generation-by-generation chain of fossils. We will undoubtedly continue to find many more fossils that will show a longer line of changes but we'll never have an example from every generation.
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