Logitech uses (or at least used to use) the term “middle button” (as Anssi mentioned) for the function. Although I managed to get my Logitech MX mouse working (this was a while ago) this way, the feel of the wheel button was never right for SU. Eventually, I went with a Microsoft Explorer 3 mouse, which has very good wheel button response, and more recently with the Razer Death Adder (stupid name; excellent mouse). I use the standard Windows driver for rock solid performance.
-Gully