Typically, each book in a trilogy or series sells fewer copies than the one preceding it. A publisher choosing not to continue the series is almost certainly making a sales decision. In a way, that works in your favor because then they may be willing to revert the rights for the first two books back to you, freeing you up to shop the full set to other publishers. (Those others won’t buy the third book when the first two are on someone else’s list.) Request the reversion so you can try this. If you get no bites because publishers are skittish about the books’ sales record, consider self-publishing. You’ve done all the writing work, and the first two stories are surely strong, they just didn’t sell. It happens. Hire a freelance editor for the final book to make sure it’s as strong as the other edited books, perhaps repackage the series with new, professionally designed covers, and then, when you promote, make a big deal about this being the complete series.