GNU Emacs Manual - Menu Mouse Clicks

Mouse clicks modified with the CONTROL bring up menus.

C-Mouse-1: This menu is for specifying the frame's default font.
C-Mouse-2: This menu is for specifying faces and other text properties for editing formatted text. See Formatted Text.
C-Mouse-3: This menu is mode-specific. For most modes, this menu has the same items as all the mode-specific menu bar menus put together. Some modes may specify a different menu for this button.(Some systems use Mouse-3 for a mode-specific menu. We took a survey of users, and found they preferred to keep Mouse-3 for selecting and killing regions. Hence the decision to use C-Mouse-3 for this menu.)