Not much has changed between the Donkey Kong Country games as far as gameplay goes. If you can pick up and play Part 1, you can pick up and play Parts 2 and 3 without any fuss or muss. They've tweaked a few things here and there, tossed in some tougher enemies, but all in all, you've got three incredibly similar games. Don't get me wrong, now: that ain't a bad thing, especially considering how fantabulous the first game was. My only qualm (besides my own use of the word 'qualm') is that now BOTH of our series stars, Donkey & Diddy, aren't present at all. That's not making a whole lot of sense to me. That'd be like if in Super Mario Bros. 3, you could only play as Toad or Toadette.
One thing I'm curious about: I wonder why none of these games ever decided to do any cross-promoting with Chikita Banana or something. Think of all of the extra money they'd make! I'd love to pick up a bunch of bananas at the grocery store and see stickers on them saying "Play Donkey Kong Country on SNES!"