Are the generated PDFs "PDF/X-3" (this is what both Lulu and Blurb are demanding for using their PDF-to-print workflow)?

We're not sure if they're "PDF/X-3", but we do know that authors have printed and sold Leanpub PDFs on Lulu. See this book for an example of a Leanpub book on Lulu (the corresponding Leanpub book is here).