For a third button you can get BetterTouchTool[1] and set up an event for the Tackpad under "Global". I have tapping the top right of the track pad (about a 1cm square) trigger the third mouse button. Perhaps it's not as great as a real middle mouse button (can't middle drag, real pain for GIS and CAD software) but it does handle opening links in new tabs and closing tabs via middle click.

[1]: http://www.boastr.net/