You're absolutely right, there's nothing magical about the 3 bars. It is just one small component of my own trading rules.
For example, when I use 3 charts to monitor "the patient's condition", I'm looking at short, medium and long term views. As I see a trade setup develop certain criteria must be met before I'll enter. This is just one of the criteria and I've found it's helpful to keep me out of bad trades. The reason is you want to see the market commit to a direction before entering a trade.