OK, a ten foot square, but... Here's the situation. We have a warpriest and a paladin and her large dog mount going down a 15 foot wide corridor that is too low for the paladin to ride. Can they all walk abreast? After all, if the paladin can ride her mount, she ought to be able to share its area.
Clearly, they would hamper or block each other in certain situations. If, for instance, they were warpriest - dog - paladin, and the warpriest was driven back by an enemy who moved up next to the dog, then the paladin is completely blocked from striking at him. (Of course, she couldn't anyway unless using a reach weapon as the enemy is ten feet away). Or if an enemy came at them from behind, the dog couldn't turn around without trampling one of the others.
I would think all three could attack an enemy directly in front of them.
But, as has often been noted, this is a game, not reality. So what do the rules say?