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.
You can click, or emulate a double click with two fingers. No third button emulation that I have worked out (and no, using your other hand on the keyboard does not count). As far as I am concerned that is a big disadvantage not having the paste buffer functionality available for the track-pad.