How do you hang three different sizes pictures as a set?

First, I love that you are using an odd number of items in your set. Odd numbers are definitely preferred over even numbers when referring to accent items in decorating.

Second, my preferred method of hanging different sized pictures is to think of a sideways triangle. The largest picture would go in the top left hand corner of your "triangle". The second largest item would go in the bottom left hand corner of your "triangle", however (and this is the most important part) hang the picture halfway in under the top picture. The smallest item forms the point of the "triangle".