Even though the post may be a partial slam; I do agree the most important thing when selecting a helmet is fit.

Now, for those who do decide to buy and wear a helmet, you have more size choices than just 3. Each helmet manufacturer tends to size their helmets slightly different. A small Giro is a different fit from a small Bell (even though Giro and Bell are owned by the same parent company). Even sizing from the same manufacturer may vary for different models. With at least 10 helmet manufacturers, plus all the different models, there is a helmet out there that will be a near perfect fit. You just have to be willing to accept the design and colors for the particular models that have that near perfect fit.