Something that may be of interest is Chrome does support 3 finger swipe down, but it's semi-hidden; and not what you're looking for. But it's way cool. The way to access it is by simply typing about:flags into your address bar. You'll see the option at the top. It's called:
Swipe down with three fingers on your trackpad to see an overview of all your tabs. Click on a thumbnail to select it. Works great in fullscreen mode.
So this suggests that they will not be doing what you would like them to do. It would obviously conflict with Tab Overview. So you should look into an app like JiTouch if you want to get more customized gesture support. It will do what you're looking for.
My guess is that Google's next step will be to map fullscreen to 3 finger swipe up; especially given the emphasis Lion will be putting on fullscreen mode apps.