MGS 2 sucks, it's a crappy story and a crappy environment. It's a Metal Gear Solid, but doesn't improve much on the existing gameplay.

MGS 3 is awesome. The story isn't great, but interesting since it's set before the other games. The gameplay is very fun, since the jungle environment is rich and full of innovative quirks to play around with. Bees on your face bitch! Also, the introduction of the Medkit and Camo systems are pretty cool new things, and I love catching your own food.

Metal Gear Solid 2: Substance has the old game, a couple not so hot Easter Eggs, but one big, big addition: about 500 stand-alone missions (some VR, others situated in the game environment) and "Snake Tales" a short series of tasks without a great deal of story, also set in the Big Shell (aka lame) environment.

Metal Gear Solid 3: Substance is the original game plus the first two Metal Gear games ever (these are oldies boys), a very handy first-person camera mode for the normal game and online play (up to 8 players, 5 or so kinds of games). It's worth buying Substance 3 just for the new camera mode, it makes the game much less restrictive and really opens up the awesome interactive environment.

edit - goddamn beat me to the punch NM...