8” round to 11x15” rectangle cake conversion

Hi bakers! I have a chocolate cake recipe that makes 2 8” round cake layers. However, I want to make a 2 layer 11x15” sheet cake. How much should I increase my recipe? I read another post where someone said batter for 3 8” rounds would fill 1 11x15”. Does this sound right? In that case do I triple my recipe? Thanks so much!!