The answer (of jokerdino) you have accepted is not an answer!! I keep on using synapse with alt+f3 shortcut till very this day, both on ubuntu 12.04 and 13.10, now even on linux mint 16 (KWin is blocking htis shortcut).
The solution is simple but tricky! :) I took me a while to get there! :)
Step 1. Go to the System Settings -> Keyboard -> Shortcuts.
Step 2. Select Windows at the list on left side of the window. You will have a many unused events with trigger: disabled.
Step 3. Assign ALT+F3 to any of this disabled options and if any alert window will jump out it should tell you witch event is blocking this shorcut (on linux mint 16 it did but on not clean version of ubuntu 13.10, where i just created new user for this test there was no alert and i could simply assing this shorcut of ours :) )
Step 4. DISABLE it by selecting the row (the shorcut should change to "assing new" or something like that) and press backspace to make it disabled again!
Step 5. You're now free to assign this in your Synapse settings :)
So to this just after you've done all of 5 steps.
Caution: in ubuntu 12.04 i remember that i had to install compiz manager, but i don't think that this is necessary anymore with this sollution. I would be glad if someone could check it for the sake of all of us! . :)