There are several ways of developing trilogies:
- Simple Segmented Trilogy -- Basically, take your storyform and break the storyweaving into three parts. The transitions between books are generally at Overall Story transitions, and frequently combined with one or more transitions in the other throughlines. This works for fairly simple stories, such as children's stories or novellas. Advantage: Easy-to-follow story development. Disadvantage: Books do not stand well on their own as complete story.
- Multi-Story Segmented Trilogy -- Blend two or more storyforms (such as those described in Simple Segmented Trilogy) and break the storyweaving into three parts. Exploring more than one storyform at a time gives you a LOT of latitude (and material) for pacing each segment of the trilogy. The Lord of the Rings trilogy loosely fits this category. Each of the stories introduced in volume 1 develops in volume 2 and resolves in book 3. The pacing of each story, however, can be radically different for each throughline. This is an advantage to this form. This works for complex stories. Advantage: Multiple stories allows for interesting storyweaving with many opportunities for "cliff-hangars." Disadvantage: Books do not stand well on their own as complete story.
- Single-Story Trilogy -- Take three stories and put them together in sequence. This is, essentially, a trilogy consisting of a core story and two sequels. Each book is a complete storyform. There are many tricks to link the books together, but the books are separate and stand alone. This works well for episodic stories. Advantage: Each story stands on its own. Disadvantage: Less compelling story connection between "episodes."
- Combination Trilogy -- By far the most complex form, this combines Single-Story Trilogy stories with either a Simple Segmented Trilogy or a Multi-Story Segmented Trilogy. Each book will have a complete, stand alone story that begins, develops, and concludes within the confines of the book. It will also have elements that span the full length of the trilogy, each book exploring a part of the whole storyform. When this works, it's amazing. It's very tough to do because you have to develop a LOT of material for each book. Advantage: Each book stands alone but also encourages reading the sequel. Disadvantage: Can be too complex for some audiences and authors, particularly during the "set-up" period in book 1.
Of course, these are generalizations. Many works exist somewhere between them.
In case you're interested, the fantasy trilogy The Bronze Canticles was written by Tracy Hickman (New York Times best-selling author). Mr. Hickman is both a Dramatica user and very interactive with his readers. I'm sure he'd be open to answering some of these questions himself.