3.5 is (as you'd expect) a revised edition of 3.0; Both are covered by the OGL 1.0 (which was created for the release of 3.0).

3.5 was an update released for balance reasons, as I understand it and didn't change core gameplay - it removed some of the more broken exploits, but you can use 3.0 material with 3.5 with relatively few, if any changes: most of the balance problem was player side.

I used to play basic D&D as a child, and then sort of ignored the whole genre until around 2005 when I picked it back up again, using 3.5, so I'm not an expert on it by any means.