The thing about cats… is that they are territorial. And yes, we mean in the larger sense - "this house is mine". But also… in the smaller sense. We mean that, sometimes, cats claim things as their own that are not, in fact, their own. That's what leads cats to steal food off your plate without any shame. It's what leads cats to sit on your laptops without any care for the electronics' well-being. It's what leads cats to "stealing" dogs' beds. Because they don't think of these things as stealing. They have simply, without anyone's knowledge, called dibs on those things, and so those things are, in fact, theirs.
But what happens when three cats decide that one bed is their own? Of course, they don't share that knowledge with one another. They call dibs in their hearts, not with their vocal chords. What happens is that they encounter a problem, which can, apparently, only be solved by attempting to squeeze into the claimed bed together, all at once.