You need to set MBUTTONPAN to 0. In CUI, you should create a new command named something like 'Zoom Window button'. The macro should be -- 'zoom;w;\
The macro above will take the cursor point when the mouse button is clicked as the first point of the window. If you don't want that then remove the \
In CUI, open the Mouse Buttons section, then the Click section. Now drag the new command 'Zoom Window button' to Button 3.
And
If you would like to assign a macro to your rt. mouse button, you can to set SHORTCUTMENU to zero (or uncheck the checkbox in Tools >options > User Preferences > Shortcut Menus in Drawing Areas).
Cheers!