[Solved] Different number of note lines after section break?

see: https://musescore.org/en/handbook/staff-type-change

It's not really clear what you mean by "note lines" here. Maybe you mean, staves? Like the first piece requires one treble and two bass clef staves (or two treble and one bass)? If so, add the staff and add the notes where you need them. Then right-click the staff, Staff Properties, and set "Hide when empty" to "Always". Now the staff will disappear on the systems where it is not needed.

I'm guessing you're looking for the style option to hide staves when empty. And possibly set the staff-properties for some of your instruments to "never hide".

You wrote:
The first one is 5 bars, the second is 12 bars, and the last one is 10 bars. Because they are so short and belong together I want to write them on one page.
The first one requires 3 note lines, while the other two only require 2.

Like this?
3_Easy_Pieces.mscz
Open the score and look at Format -> Style -> Score -> Hide empty staves within systems
and also 'Don't hide empty staves in first system'

I keep getting the 3 bars from the first piece.
That's why 'Don't hide empty staves in first system' should not be checked.