As you see in the picture, we have 3 mattresses and 2 cracks in the middle that extends from the upper area to the bottom area. What are these cracks called? Is there a specific word for it?
There is no specific name for this, but things that fall into these cracks are said to be behind, beneath, under, between, in, or underneath the cushions.
If you're ever short of money remember there's always loose change beneath the sofa cushions. Always.
(Edit) If you do search for examples to see which of these might be more prevalent, try not to make your search too specific. For example, I found this headline (among many others) from a search for "beneath the cushions":
this from "under the cushions":
and this from "between the cushions":
Since all these seem to mean more or less the same thing, which you should use depends on which sounds best under the circumstances.