Yes, Aperture 3 could even open Aperture 1 libraries.
See the Aperture 3.5.1: Release notes http://support.apple.com/kb/TS2518
Scroll down to the bottom of the document: "A note on Upgrading".
I haven't purchased Aperture 3 yet as the big question is will Aperture 3 recognise an Aperture 2 library, but from what you are saying you sound confident Aperture 3 will import the old library?
You don't need to import the library, just open it in Aperture 3, and let Aperture upgrade it. What kind of backup do you have? If it is a Time Machine backup, restore the library from Time Machine, and then try to open it.
I don't know, if Aperture 3 can restore Aperture 2 vaults.
If your backup is a copy of the Aperture 2 library, simply use Aperture's "File > Switch to library" to open the Aperture 2 library in Aperture 3.5.1.