BetterTouchTool: Is it possible to use a context-menu item with a gesture

Remote Buddy will allow you do to this (plus a whole lot more if you wish). You can customise a range of actions for each application and assign these to the Apple remote buttons (this includes both press and press-and-hold). You can also define your own custom actions if the built-in ones don't suit. Nothing came close for me in terms of flexibility and ease of use - I tried both Mira and Sofa Control before settling on RB.

Personally, I use this to control my XBMC media centre with a range of custom button-presses and use the launch menu to start it up. It works with the white plastic remote too, I've just got the aluminium one thats shown in the screenshot.

Use this AppleScript to open a file with a particular application:

set appPath to path to application "TextWrangler"
tell application "Finder"
    set sel to selection
    if sel is not {} then
        set filepath to item 1 of sel as text --> The path to the one selected item.
        open file filepath using appPath
    end if
end tell

I've never used BetterTouchTool but according to the documentation, you can set up any input (such as a three-finger tap) to run an arbitrary AppleScript if it's saved as a scpt file.

Open the AppleScript Editor, copy and paste the code above, and save it as opentw.scpt or something similar. Then in BetterTouchTool, add a new gesture. Choose three-finger tap as the gesture, and choose the Predefined Action of Open Application/File/Script. Locate the AppleScript file you saved above as the script to open.