This interview with Margaret Clark states that the Romulan War was to be a 3-book series:
http://trekmovie.com/2007/03/07/new-novels-to-mine-trek-history/
But that's in the interviewer's words, not Margaret's. It's possible Margaret just said she was considering the possibility of doing it as three books, and the interviewer jumped to the conclusion that it was a sure thing. I mean, it's from four years ago. Any plans that far in advance are bound to be tentative. Remember when A Time to... was going to be 12 books and Typhon Pact was going to be 6?
Yes, but my point in posting that is that there *is* out there something (attributed to then-editor Clark, no less) that stated the Romulan War was supposed to be a trilogy. I had read that same interview years ago and it always stuck in my head that it was supposed to be a trilogy. Others could have read it as well and been left with the same impression.