To get 3-finger swipe forward/back, change the Spaces gesture to 4-finger swipe, and the Page Turning gesture to 3-finger swipe. This works system wide (seems to invoke ⌘-[ and ⌘-]) so not only do you get browser-back/forward but also in the Finder, etc.

Before Lion I used BetterTouchTool to enable full-gestures on my Non-unibody MacBook Pro, and I notice Lion requires a bit more deliberateness for gestures.

Also, who the heck invented 3-finger pinch? I can’t even do it.