romantic/erotic story. I am wondering if the scope is too big for one novel and whether it would better fit into a trilogy.
It's not so much the scope or structure of the story that calls for separate volumes. It's the word count.
You'll get a much better handle on your final word count when you start putting your story down on paper. Perhaps all three of your stories stories will combine to less than 90 or 100K. If that's the case you'd have one conveniently-sized book with three sections.