And this just seems all kind of wrong…
Can you share the file here?
If you’re asking why the number of staves in page view doesn’t change to add the 3rd stave in the second divisi change, see the note on this page: “The number of staves in a system is defined by the first divisi change in the system. If you input a divisi change on a system with an existing divisi change, the number of staves is not changed to reflect the new divisi change until the next system.”
Basically, if you put in a system break at the position of your second divisi change where you add the 3rd staff, it’ll appear at the start of the new system. however, the 3 staves will continue until the next system. any unison material is automatically duplicated onto the bottom staff until it goes away on the next system.