Downloading and installing Numbers 3 does not remove Numbers '09 v2.3 from your computer, something you may be thankful for as you discover the current differences between these versions.
Look in the applications folder on your Mac, where you should now find both versions (one may be enclosed in a folder named iWork '09). You can distinguish between them by selecting one and pressing command-I ('eye', not 'one') to Get Info and read the version information in the Info window that opens.
A more direct method of distinguishing between the two is to look at their icons. Both show a three dimensional bar chart, but version 2's chart has three vertica bars, version 3's has four. Drag the version 3 icon to the dock , and you will be able to launch it directly from there or by dragging a file to the dock and dropping it on the icon for the version you want to open it.
You can also select a Numbers document, Get Info on it, and change the version to open that document using th Open with... section of the Info window. UNTIL YOU ARE CERTAIN THAT YOU WANT TO USE ONLY NUMBERS 3, AND DO NOT NEED FEATURES SUPPORTED IN NUMBERS '09 v2, I WOULD STRONGLY RECOMMEND NOT APPLYING THIS CHANGE TO ALL NUMBERS DOCUMENTS.
Several features supported in Numbers '09 are NOT supported in Numbers 3 at present. Most changes are covered in the three threads linked below. The most current lists in the first two are on the last page or second last page.