What's new in v3?

Hmm - the original announcement of V3 contained a link to the changes from V2 ... http://www.codeigniter.com/userguide3/changelog.html
The list of changes is quite extensive!

As to the 'tab' dropdown - you might have noticed the menu icon on the top right of the user guide pages ... mousing over it reveals "classic layout", and clicking on it toggles from the sidebar menu to what I think you mean by the 'tab' dropdown menu layout.

The closest thing to an overview of the changes is the bulleted list at the top of the V3 announcement ... http://forum.codeigniter.com/thread-1657.html