It works just the same on the V3.
So theoretically they could add cbr support simply by adding the file extension to the book shelf and treat the cbr file as a book, with the last viewed picture as the bookmark. Zip support isn't fully implemented, but cbz support should theoretically be equally easy to add.
Disclaimer: I've never seen the V3 firmware, so the last paragraph is pure speculation on my part. Renaming cbr files to rar should make it possible to use them on the V3, though.