I currently run LR 1.41, 2.7, 3.0, and 3.2 RC on my machine. *Mainly because I teach Lightroom and provide forum support various places*
I would never attempt to open a different version of Lightroom without closing the open version. If you have already purchased LR3, I would just dive in and leave 2.x behind. The new world is not so different that you would get lost (except the import module)