When you start Lightroom 3, it’s going to want to upgrade your catalog. Gasp! Don’t worry, it’s painless. In fact, it leaves the original catalog intact, so you don’t have to worry about it.
You’ll get this dialog. If this isn’t the catalog you want, then choose the correct one using ‘Choose a Different Catalog’. Click Upgrade. You’ll then see:
If you’ve plugins active that require a catalog update, you’ll see this:
If not, then let it run. The upgraded catalog will then open in Lightroom 3. Congrats.