Three is a hugely significant number to humans whether we realize it or not. We live on the 3rd rock from the sun. The significance of three when it comes to putting things in order - there are no prizes for 4th place. Goldilock and the three bears. Triangular engineering structures are the strongest. The Great Pyramids. etc etc. However we look at it, the number three is hardwired into our brains.
In design the number three is also crucial - whether it's putting plants in the garden or arranging food on a plate three of something often looks better than any other number and of course photography is no exception - for example the rule of thirds in composing an image.
Portraits too work with three people providing an interesting and more dynamic option to an image of a single individual. Lighting is pretty straightforward too - in all these 'three' images a single speedlight was used through either a softbox or shoot through umbrella held off at 45 degrees giving reasonable soft light on all the subjects. Combine this with the sun as a backlight and you can end up with nice well lit images.