Emacs uses the symbol mouse-1 for the first mouse button, while XEmacs uses the symbol button1. XEmacs still allows the use of the Emacs symbols for backwards compatibility. Therefore the following will also work for both emacsen:
(global-set-key (kbd "S-<mouse-3>") 'apropos)
For those interested in ComparativeEmacsology – the expression (kbd “S-<mouse-3>”) yields [S-mouse-3] on Emacs, and [(shift mouse-3)] on XEmacs.
Note however, that for Emacs, a binding for “<S-down-mouse-1>” takes precedence over a binding for “S-mouse-1”. Always use C-h k to check whether the “down” event is already bound, if you find that a set-key statement seems to not work at all. Example: Per default “<S-down-mouse-1>” is bound to mouse-set-font. The following has no effect:
(global-set-key (kbd "S-<mouse-1>") 'apropos)
You also need to unset the down event:
(global-unset-key (kbd "S-<down-mouse-1>"))