"The great thing about Harry, Ron and Hermione is that they are all the uncomfortable third friend.”

I like this explanation. And I think it's true. Harry, Ron and Hermione are all in some ways a third wheel and overall they balance each other out well.

Each of them brings a key component to the relationship. Harry of course is the leader who takes control and usually inspires a degree of loyalty and courage. Hermione is of course the brains , and supplies a healthy dose of pragmatism and common sense. And finally Ron is the heart, he’s in my opinion the most soft hearted of the three. Harry and Hermione are a bit more ruthless and are prone to more violent reactions. Ron supplies a necessary softness that keeps Hermione and Harry from going off the deep end.