Many people like to mix single flavors until they have the right percentage for themselves, then decide the ratios of the flavorings. This way you can see which one you prefer up front, the strength of the flavors and so on. There is nothing wrong with trying a mix of the 3. Sometimes we get lucky and nail it on the first try, lol.
What worked best for me was to try out other people's recipes (posted in this forum). That taught me a lot about flavorings and how they work together, how they compliment each other, and whether or not I even liked them. Also, a ballpark of percentages of how to use a given flavor. Then I got comfortable at mixing something of my own.
It's a lot of fun. Can be addicting! Definitely read some of the tips in the DIY forum, ask lots of questions, follow safety guidelines... have fun with it. We all help each other here. It's an awesome forum.