Karen Wiesner in her book From First Draft To Finished Novel calls the third step Blueprint. In this step there are a number of levels. This becomes an outline and includes a number of features.
Character sketch begin the process for her. For me the first step is designing the plot in a very general way. Much depends on if you're a plot person or a character person.
Let's look at the character sketch. This means writing down everything you can think of about the character. For me I choose a character's Sun sigh, Moon and Ascendant. This gives me a number of character points. Some go together and some are in opposition giving the character questions about what they want and why they want it. I generally don't really know my characters until I finish a rough draft of the story.
Plot Sketch is her second point. As you saw I'm the reverse. The plot sketch depends on the type of story being written. A mystery has one kind of plot. A romance a different one and there are many variations. I decide on the beginning and the end. The middle will come as the rough draft is written.
Then there's the Setting sketch and this can be a minimal or as worked out as needed. This for me is world building and the world may be contemporary, historical or fantasy. Al;l the features here are included even the houses,
These three sketches form the blueprint of the plan of the story you;'re creating.