Third Assistant Director
1. (Third or 3rd AD) works on set with the “First” and may liaise with the “Second” to move actors from base camp (the area containing the production, cast, and hair and makeup trailers), organize crowd scenes, and supervise one or more production assistants (PA). There is sometimes no clear distinction between a 2AD and a 3AD. Although some industry bodies (American DGA) have defined the roles in an objective way, others believe it to be a subjective distinction.
Source: Wikipedia