Your best bet is to ask before you buy.
It depends on the reason the book was released in the EPUB 3 format. If it's a static text, and the publisher was only trying to follow the latest guidelines and standards, they may have added the extra bits required by the older standard, so it would degrade gracefully and be available to more customers.
If, on the other hand, the book was released as an EPUB 3 in order to take full advantage of the standard's dynamics and interactivity, it's likely that the publisher didn't add the required legacy information. They might have pegged a particular device, to ensure that the book would function predictably. For example, Walrus Books released H.P. Lovecraft's novel, "Kadath", for the iBooks app in EPUB 3. It is highly interactive and intended for the iPad. They had no reason to make it work on other devices, and probably didn't bother with legacy shims.