How many mattresses does SpongeBob sleep on?

It’s two mattresses on top of a box spring. In the pickles episode from season one, he’s trying to remember the proper order of his bed. In listing all the layers and possible combinations, he only includes two mattresses, leading us to believe the third white rectangle on the bottom of his bed is indeed a box spring and not a third mattress. “It’s mattress, SpongeBob, mattress, then sheets, pillow!”