Freedom 7 : the historic flight of Alan B. Shepard, Jr. / Colin Burgess. (OCLC #857972968)
Call numbers with three cutters are pretty unusual in the Library of Congress classification schedule; I mainly see them in G (maps).
If you are using a class number that already has two cutters specified, such as TL789.8.U6A5 (Space travel, United States, Apollo Program), you still need to extend the call number to sort items by main entry. Rather than adding a third cutter (starting with another letter) just extend the second cutter with numbers.
For this title (about a mission from Project Mercury), I started by identifying the second cutter used for that project (M4) and checking the LC catalog to shelflist, ordering by main entry. Library of Congress already has a title in their collection about this topic by this author (“Selecting the Mercury seven”) with call number:
TL789.8.U6 M428 2011
so I shifted the cutter yet again to also order by title:
TL789.8.U6 M427 2014