I think "Kanapka jest chleb, ser i mieso" is also a valid translation?
1) you need instrumental case in noun+jest+noun sentence,
2)It would be Kanapka jest chlebem , mięsem i serem but this sentence sounds really weird to me.
Thank you for the clarification - or the reminder! I forgot about the instrumental in this situation.
(I've just tried it out on a couple of native speakers, and they said it sounded ok. One actually translated the sentence this way).