The Third Legion can refer any of these Roman Legions:
Legio III Augusta Legio III Cyrenaica Legio III Diocletiana Legio III Gallica Legio III Isaura Legio III Italica Legio III Parthica
It can also refer to the Third Legion (Syria) of the Syrian National Army.
See also 3rd Legion Tercio "Don Juan de Austria" regiment of the Spanish Legion 3rd Legion First Encirclement Campaign against Jiangxi Soviet 3rd Legion of the Vistula (Poles) Battle of Alcañiz