User blog comment:B214/Season 3 confirmed!!!!!!/@comment-179.232.68.37-20141214002810/@comment-1359322-20141214094656

That parts explains how Ise obtained the Triana, like Sinbad said, not a good choice to skip it. It'll leave a hole in the plot.

So after Season 3, if they do continue with a Season 4 & 5. It'll most likely be
 * Volume 7, 8 & 9 (Season 4) - 12/13 episodes
 * Volume 10, 11 & 12 (Season 5) - 12/13 episodes.

I'm sure people is going to switch to complaining that Volume 11 & 12 is too long, but there are other LN that gets worst treatment like Date A Live, the anime producers covered 3 volumes in 10 episodes for Season 2.

So instead of complaining over minor issue like there's too little time to animate those volume, be thankful that TNK is giving DxD justice compared to series like Black Bullet, Date A Live, Oda Nobuna and Madan something Vanadis.

P.s. For those that feel like this sounds similar to a comment from an anon below, you're right. I took the points from him.