As much as I love 3-speeds, I would choose another bike to tour on. I agree with those above - pick a cheap and available rigid mtb. Here's my '88 Trek 830. It's 7lbs lighter than my 3-speed. You could probably get this for $100. I paid $500 back in '88. A trek 930 is slightly better with regard to components, but this bike is bombproof enough and has as many hardpoints as my Trek 520, a dedicated tour bike.
http://i1.tinypic.com/23uoniv.jpg