Yes. Provided you move the old app to a folder within Applications first, upgrading will leave the existing app in place. Then you can use either.
However... the file format used by Numbers 3.x is not compatibile with Numbers 2.3. So unless you export files back to Numbers '09 format (File / Export / Numbers '09) you won't be able to open any that you work on in Numbers 3 using Numbers 2.3
I'm not sure if there are any 'roundtrip' problems resulting from this - but it is clear that such roundtrips in Pages results in some things getting lost along the way (relating to features from Pages 09 that are not in the new app)