Why I have 3 different Java versions on my Mac?
Different applications will link to different Java libraries. Older apps will most likely require Java 6 (1.6.0_65), whereas newer apps will link to newer builds of Java respectively.

By way of example, I know that the Mobile Connect app for my old Huawei 3G modem requires Java 6 to be installed, Adobe Photoshop CS6 requires Java 7 (IIRC) and your browsers will most likely request you to run the latest Java, which is Java 8.