Apart from the dogs owner who SHOULD be all of the dogs ultimate leader, it all depends on the dogs genetic temperaments as to which one is the "top dog". Its the most dominant and assertive dog of a pack, that is likely to be top dog.
In MY opinion, this is usually an intact male, who will allow a female dog more privileges than it will to another male.
Its not true that females usually "rule" its just that a dominant male will be a lot more lenient with a female and "allow" her to answer him back if he is so inclined.
But of course a female CAN be a pack leader if she is more dominant natured than the rest of the packs dogs.
Size and breed can be nothing to do with what dog is leader,and a very dominant Chihuahua can be higher in status than a more submissive Gt. Dane.
Although size can help sometimes with dogs confidence in life.
A typical male leader dog is bold, assertive, calm, very confident and focused and only needs to have that "look" and "stance" to convey his status. He does not need to fight to show his superiority, just his smell and body posture show this clearly. But he WILL fight if openly challenged and he usually comes out on top.
In a pack of 3 dogs where none of them have a dominant nature, the most bold and outgoing dog will probably be the leader, after the human owner of course.
Dominant dogs are fairly common, but TRUE Alpha dogs that will even challenge a human owner for top status are "rare" and do not make good pets. They need professional handling and can be dangerous for the average pet dog owner. Such dogs, If of a large impressive breed are often used as military or security dogs.
GSD owner for 56 years.UK.