The third class [noun] is the sophomore class. As a noun, it is two separate words. As an adjective, it is hyphenated.
The third-class [adjective] cadet was early to class.
Do not use numerals when referencing.
third-class cadets
See Fourth-Class System.
title
Only capitalized when it precedes a person’s name and becomes part of that person’s title.
Governor Nikki Haley thanked The Citadel for its support.
Joseph P. Riley, Jr., the former mayor of Charleston, accepted the newly-created chair in his name.