i started a new blog and i see that there are two pages it starts out with:
i want to add a third one but instead of having it load up an internal page, i want to have this go to an external link. is this possible ?
Admin Dashboard -> Appearance -> Menus
If the theme is WP 3.0 ready, you will have an option to add a custom link (ie external link) on the left side, and you can drag and drop it into the order you want.
Justin Tadlock wrote an awesome post about 3.0 menus - http://justintadlock.com/archives/2010/06/01/goodbye-headaches-hello-menus
If your theme isn't 3.0 ready, I believe you can still use the custom menu widget.
If memory serves, look under the pages. If not, somewhere else in the menu editor. Somewhere in there youve the possibility to enter an arbitrary URL.
If your Theme is not 'Menu' ready, I suggest you to use my plugin 'Redirector' http://wordpress.org/extend/plugins/redirector/ After activating the plugin you have a new meta box on the 'page' screen. There you can select any of your pages or an external url