Acrobat 3 contained two sets of three toolbar buttons; Page Mode and Page View.
Page Mode contained the following three buttons:
None, Show Bookmarks and Show Thumbnails
Page View contained the following three buttons:
Fit Page, Fit Visible & Zoom 100%
To save space on the toolbar; TriState was built to convert groups of three toolbar buttons into single tri-state buttons.
This source demonstrates the use of the following notifications:
In addition, it also demonstrates adding and removing toolbar buttons.