3rd class as in the 3rd class who skills you can still use while playing the job. I know you only need 2 to unlock. They are listed in the link you gave
Paladin-requires Gladiator level 30&Conjurer level 15 Can use Gladiator, Marauder and Conjurer
Dragoon-requires Lancer level 30&Marauder level 15 Can use Lancer, Pugilist and Marauder
Monk-requires Pugilist level 30&Lancer level 15 Can use Lancer, Pugilist and Marauder
Bard-requires Archer level 30&Pugilist level 15 Can use Archer, Lancer and Pugilist
Warrior-requires Marauder level 30&Gladiator level 15 Can use Gladiator, Marauder and Pugilist
White Mage-requires Conjurer level 30&Arcanist level 15. Can use Arcanist, Thaumaturge and Conjurer
Black Mage-requires Thaumaturge level 30&Archer level 15. Can use Thaumaturge, Archer and Arcanist
Summoner-requires Arcanist level 30&Thaumaturge level 15. Can use Archer, Arcanist and Thaumaturge
Scholar-requires Arcanist level 30&Conjurer level 15. Can use Conjurer, Arcanist and Thaumaturge