To answer the question I am not aware of any existing 3.0 forums but I'm told it looked very muc like 3.6.x which is what this forum is built on.
The 3 series skipped from 3.0.x right to 3.5.x, the 0.5 was added when the hooks system was added to the product. Prior to 3.5 any modifications you made you had to edit the original php files so it was a major advance.
The oldest VB forum I am aware of in operation is VB 2.2.9, although it has been heavily customized - http://forums.somethingawful.com/ - Be warned, it may not be a "politically correct" site.