User blog comment:B214/Season 3 confirmed!!!!!!/@comment-24502975-20150114150539/@comment-24502975-20150115001947

I don't think it has anything to do with the OP, it's just there to be there. The PV scenes were created exclusively for showing off DxD BorN, although Rose's sequence would make for good OP-material I'm expecting something even better.

About the Dragon, you can't say it's not part of the main story. Ise vs Sairaorg isn't part of s3, but Sairaorg is. So obviously showing that Dragon was meant to be a display of someone/something.