The use of groups of three is common in design in many forms, in interior design items are often grouped in threes, in scrapbooking and photography we are encouraged to consider the rule of thirds because we naturally find things pleasing to look at when the image contains interesting features at the points where the thirds meet.
In addition to all of the above reasons, I like using three photos because where one shot can give a snapshot, with three photos you can start to tell a story. Anyone looking at the layout can see what is happening over the space of a longer time, it maybe a few seconds if the shots were taken rapidly one after another or it could be the highlights of a whole day.
By changing the products used, journaling and title styles and making adjustments such as angling elements you can make several layouts using essentially the same design but which end up looking quite different from one another.
If you have any layouts using a design similar to this that you would care to share please do leave us a link to your blog or online gallery in the comments section - we would love to take a look!