Yesterday, I learned these weren't the same thing. Well, I knew about the viewpoint character already, but I always considered the other two terms to be the same. Orson Scott Card establishes a clear distinction between Main Character and Protagonist, however, which I believe is great to keep in mind.
Yes, most of the time, you'll want your MC to be your protagonist. But this doesn't have to be the case, and it can enhance your story to make them different. So what's the difference?
Protagonist: The protagonist is the character we're rooting for. He has our sympathy. We agree with his goals and we want him to win by the end of the novel. If your MC is a hero type, he'll be the protagonist, too. Star Wars' protagonists are Luke, Leia and Han Solo.
Main Character: The Main Character is the character whose actions are driving the plot. His character evolution and final decisions will decide of the novel's end. He has everyone's fate in his hands. If your protagonist is proactive, he's likely to be the main character, too.
Sometimes, however, the main character is the villain. This is true for Star Wars. Luke and co. spend the first two movies reacting to what Darth Vader does. And what is the third's climax? When Vader chooses his son over the Empire.
Viewpoint Character: Sometimes it's best when the viewpoint character is neither the protagonist nor the main character. In mysteries, for example, the POV character is often a sidekick, because the detective knows the murderer a fair bit before the climax (think Doctor Watson and Sherlock here). Most of the time it's advisable to keep the POV on either the MC or the protagonist, however, as this is where the action will be.