Off the top of my head, F3 is bound to "find next" in most emulations, so you can't bind it that way.  
Go to Tools -> Options -> Keyboard -> Key Bindings.  In the box "Search By key sequence", press F3 and slick will tell you what it's bound to.  If you want to change it, clear it there first.  
on the other hand, if you enter find-in-files in the "Search By Command" box, it SHOULD tell you what keys are bound to it already.  In Visual Studio emulation, I have Ctrl+Shift+F.
Best, 
Jim.