The beauty of the three sentence logline is its structure: one sentence each for the beginning, middle and end of your novel. I can whittle almost any plot down to a single sentence, but selling a project is not always just about how few words you can use to sum up the plot.
Here's the strongest one sentence logline for The Steampunk Novel: A young magic-handler desperate to find
her Da confronts conspiracy, murder and forbidden magic under the
streets of 1888 London.
It's good. It covers the plot. It identifies the protagonist, her goal and the obstacles she faces. All in all, it does exactly what it's supposed to do. But, when I entered The Bakers Dozen Agent Auction in November I felt like I needed something that touched on the specifics of the story.
This is what I came up with: Magic-handler
Keira Fennel's search for her father intersects with shape-shifter
Lowen McCrae's hunt for a murderer in the alleys of 1888 London. When
they uncover a plot to construct a mechanical heart from stolen
flesh, Keira's skill with gears and magic makes her the target of the
otherworldly villain. She's forced to participate in the experiment
or lose both Lowen and her father.
This version is also good. It covers the plot. It identifies the protagonist, her goals and the obstacles she faces. It also provides details that highlight the uniqeness of the story and the final conflict.
Is it better than the shorter version? No. Probably not. But it is capable of serving a different function. Because of the added length it is not just a logline, it's also a very short synopsis and there are times when that property will be more important than the brevity of the shorter version.
Just like a synopsis (which should have 1-2 page and 4-10 page versions) there's no harm in having loglines of different lengths one very short one to sell the heart of your story and one slightly longer one that covers the plot arc from beginning to end. If anything, identifying the guts of each act of the novel is helpful in writing both the query letter and the normal length synopsis.
So. What are the three acts of your novel? When you distill them into a single sentence each, what does it look like?