© Boy Scouts of America 1998. Photo by Roger Morgan
This picture shows three dominant elements. Is is usually easier to compose pictures with an odd number of elements rather than an even number. This is especially the case in comparing three element verses two.
What is the problem with just two elements? Usually, there is too much empty space between them. A third element fills that space nicely. What is empty space? Space that contributes nothing to the meaning of the picture and destroys the feeling of interaction between the two elements that are there.
In making a picture of only two people, you can carefully select an angle that closes up the space between the two heads shooting over the shoulder of one into the face of another.
With three people in the picture, the problem usually goes away.