If you create items in Creo Parametrics 3.0 can you view and edit them in Creo Parametrics 2.0?
No.
(Technically there are some games you can play with GCRI, but that is still nothing like native backward compatibility.)
And just for completeness, could someone please reply to say whether the native files created in CREO 2.0 will automatically work in CREO 3.0, especially parametric files?
Thank you.
PTC products have, to the best of my knowledge, always been forward compatible so therefor, "Yes".
Correct. I know for certain that 2001, Wildfire, Wildfire 2, 3, 4, 5, Creo 1, and Creo 2 files will all open in Creo 3.
Not sure about files prior to 2001.
I've opened files that go back to release 16 or 18 (mid 1990s) in Creo 2 without issue.