Sorry it's taken almost 5 years to get back to you, dog_on_wheels, but you are correct.
Not to sound too crazy, but I think Instrumental 3 is technically an alternate version of Instrumental 2 (at least that's what the description of my copy says). You can find it here: http://bt.etree.org/details.php?id=4064
A lot of Wilco demos, including these, are available here: http://forums.viachicago.org/index.php?showtopic=26738
Where would I find it?