I agree it would have been better with two characters on Book 3. A Lit-verse character would have been nice to see in that case.
I've given up hope of seeing any more Lit-verse characters on the covers. At this point they seem to be pretty much sticking with the canon characters. I can understand that though, it's got to be a lot cheaper and easier to just use preexisting photos than to hire a model for a new character.
More importantly, I have to assume that recognizable TV and movie characters sell more books, especially to impulse buyers, librarians, people in airports, etc. It's the smart, commercial thing to do.
There's a reason you put Wolverine on the cover of an X-Man novel!