Emacs assigns bindings assuming a three button mouse. On Windows, if
a two button mouse is detected, a hack is enabled which lets you
simulate the third button by pressing both mouse buttons
simultaneously. w32-mouse-button-tolerance defines the timeout
for what is considered “simultaneous”. You can check how many
buttons Emacs thinks your mouse has with C-h v
w32-num-mouse-buttons.
If you find yourself needing the mouse-3 bindings more often than mouse-2, you can swap the buttons with the following code in your init file:
(setq w32-swap-mouse-buttons t)