In order to make sure your ePub 3 eBook works well enough with ePub 2, I suggest you download and install Adobe Digital Editions 2.0.1 and use that to test your eBook.
You can get ADE 2.0.1 for Windows or MAC (32-bit) here. Adobe took down the page, but The Wayback Machine still has it.
https://web.archive.org/web/20200619...downloads.html