The primary class for a given job has to be at least level 30 to unlock the job. It shares the same level and XP as the job, and all its class skills are available to the job (as normal skills — not cross-class).
The secondary class has to be at least level 15 to unlock the job, and it's available to pull cross-class skills from.
The tertiary class isn't required, but can be used to pull cross-class skills from in the same way as the secondary class.

Warrior: Marauder (primary) + Gladiator (secondary) + Pugilist (tertiary)
Paladin: Gladiator (primary) + Conjurer (secondary) + Marauder (tertiary)
Monk: Pugilist (primary) + Lancer (secondary) + Marauder (tertiary)
Dragoon: Lancer (primary) + Marauder (secondary) + Pugilist (tertiary)
Bard: Archer (primary) + Pugilist (secondary) + Lancer (tertiary)
White Mage: Conjurer (primary) + Arcanist (secondary) + Thaumaturge (tertiary)
Black Mage: Thaumaturge (primary) + Archer (secondary) + Arcanist (tertiary)
Summoner: Arcanist (primary) + Thaumaturge (secondary) + Archer (tertiary)
Scholar: Arcanist (primary) + Conjurer (secondary) + Thaumaturge (tertiary)