Pitching a Second/Extended Edition

So my first book was self-published to good reviews as a novella in 2013. This was followed by a full-length novel that was picked up by an Indie publisher after they checked out the first book. For the third book (of the same series) I decided to get an agent. Additionally, I learned the publisher of Book 2 was folding after I began shopping Book 3 around, and that book is now out of print. (full rights have reverted to me). The third book received some interest and encouraging remarks, but ultimately all the agents I submitted to passed (my favorite rejection: "Marvelous! Stupendous! I loved it!... But it's just not a good fit for me.").

I figure that part of the problem I have with Book 3 is agents not wanting to pick me up in the middle of a series, and would prefer to start with something new. I'm also dissatisfied with the edit of Book 2 (the publisher took total control. I especially hated the cover, but was told, "This is what we're using") and decided it needs some rewrites. However I also felt that there's a number of things I wanted to revise on Book 1, part of which included expanding it from a novella to a full-length novel both to better flesh out the story, and because a novel is an easier sell. I'm currently in the process of doing this now.

I'm curious on how I should approach pitching this to an agent once the rewrites are complete. I'm certain I'll want to be honest that the book had previously been published as a novella, but that I have full rights to it. However I'm unsure of how exactly I should broach this once it's time to start shopping it around.

Well, Book 1's rewrites aren't nearly done and I've already doubled the length, so it substantially IS a new book. All said and done, I suspect it will end up being about 66% new content greatly fleshing out a lot of stuff I abridged (it was planned to be a short story, but ended up longer).

There ARE some mitigating issues on the first two books. The first was a novella, which are a hard to sell in the first place (I know even established, big name writers have trouble selling them as standalone stories). I also only had myself for marketing (self-published) and no budget to do it with. The second I literally got NO help from the publisher whatsoever. No real marketing support, an edit I didn't like, and a very bad cover (which i rejected but they used anyway).