You could also try browsing
http://code.google.com/p/epub-samples/ for samples of ePub3 files. Look at the
Samples Listing link
http://code.google.com/p/epub-sample...SamplesListing for all the samples they have.
Each sample let you either download the ePub file or browse the source.
These samples were found by Googling for
sample epub3.