On a ferry ride, I saw these three boats slowly coming up to the starboard side and with the great clouds in the sky I knew I wanted to pull in as much of it as I could. I'm not sure exactly why, but the "rule of odd" says that for a natural and pleasing composition you should choose to have odd number of main subjects. In this case thats 3. Its not a rule I follow that closely; I've made interesting images with just 2 main subjects and sometimes 4.