They are called this way because if you could mark your trail, you'd look like you're making the number three. First, start in a T position with one foot on the back. Imagine an axis which you'll be gliding perpendicularly. Push with the leg behind and leave it straight, hanging behind, while the other is bent and tries to go back on the axis, making some sort of a semi-circle, then straighten the bent leg and push yourself backwards even with the free leg still hanging behind, all leading back to the axis. This is better done with a coach to show you how.