Rule Of Odds. Simply put, there’s a school of thought that multiple subjects in a shot are more pleasing if there are an odd number of them, as opposed to even. Think 3, 5, 7, etc.. The number “one” is technically odd, but I don’t think that’s what this “rule” is referring to.
Often an odd number of subjects creates symmetry in your shot – use an odd number of things for the viewer to focus on without making it too cluttered. If you have people subjects, go for 3 or 5; maybe food photography is your thing and you want us to check out some tacos you’ve just made. Or if you’re out and about – maybe you spot 3 birds sitting on a wire. Oh look, it’s five canoes moored together at the lake – all it takes is for you be to a touch more aware of what you see and try and train your mind to compose in odds.
Remember not to over-crowd the image with too many elements. The average human brain has a better chance of feeling attracted to arrangements of 3 or 5 elements, but as you increase that number, this rule will be less relevant.
TIPS:
- Movement: Add dynamism and a feeling of movement by composing shots with good space on the correct side, usually more space in the direction of the movement.
- Cropping: Consider using cropping in your editing software to improve the composition. When done correctly, it can improve things immensely.
- Positioning: Try to align vertical elements such as buildings, people walking on the street or a solitary tree along a gridline to generate more visual interest.
- Less Is More: 3 to 5 objects of interest in a scene usually work best. Avoid making your image cluttered.
- DoF: Use an appropriate Depth Of Field to ensure good sharpness across all your subjects.
Ritva Sillanmäki Photography 
Three is good!
Thanks Sue!