Three stage VTEC is actually a pretty simple concept. Below a certain RPM - 2000 usually - your engine only opens one intake valve. This creates a nice swirl of air in the cylinder, which is good for fuel economy. (No, don't ask me why it increases fuel economy, I have no idea. Swirling air = good.)

Above 2000 but below third stage engagement it runs like any other Civic.

The third stage is identical to conventional SOHC VTEC, just some added lift on the intake lobes.