Yes, it would - and welcome to the forum!I have a ff3 laying around, would it work on a q5
The Walther Q5 comes with three mounting plates conforming to three different mounting norms, namely Trijicon, Leupold and Docter. Your Burris Fast Fire 3 should conform to the Docter mounting norm.
To avoid repeating what others have already posted, I'll refer you to some forum threads on the topic:
Note that you might have to look around a bit to find the correct screws.