Can a MC Wizard 2/Cleric 18 learn a 3rd level spell, i.e fireball that he would find on is adventure?

@PlaguescarredCan a MC Wizard 2/Cleric 18 learn a 3rd level spell, i.e fireball that he would find on is adventure? That character wouldn’t be able to learn fireball, since he doesn’t have enough levels in the wizard class.

— Jeremy Crawford (@JeremyECrawford) August 22, 2014