|The trunk of a poison ivy vine definitely will get to 3" or wider, though it's usually relatively flat at that size (an inch thick or less). It's very easy to recognize by the hairy roots sticking out to grab onto whatever it's growing up.
If you're looking at a free-hanging vine that's round, 2" thick, and not hairy, it's probably grape or bittersweet. Grapevines hang onto branches by small, curly tendrils of the vine, and develop a shaggy bark. Bittersweet twines around branches and tightens as it grows, and has bright orange roots.