Help with expansions

Hi. I am wondering what happens if i download all 3 expansions. I currently have wotlk and if I download pandora and cataclysm, will i just run the latest expansion (cataclysm) to play all 3? Will i still have my characters? If i start the game with pandora or cataclysm, will i still be able to access the low level lands from wotlk to level my toon and play wotlk content?

You have the exact same question on this post http://forum.warmane.com/showthread.php?t=332478 and many answers already.

Very observant of you. It doesn't make sense. So basically I'm playing the really old wow, not the new... And can't use my wotlk toons on the newer expansions? (That's gay).