A Sorcerer 4/Artificer one character may have two 3rd degree spell slots, but can't Forged 3rd stage Sorcerer spells (obtained at fifth level Sorcerer). They could, nonetheless, use their 3rd level spell slots to “upcast” their known or prepared 1st and 2nd level spells for the two Sorcerer or Artificer. https://dungeon-dragons-barbarian54219.canariblogs.com/the-smart-trick-of-dungeon-dragons-bardbarian-that-no-one-is-discussing-33467433