Tri-Rocker
Tri-rocker is used by some aggressive skaters. It is another method of making grinds easier. A frame with a tri-rocker setup has only one wheel in the middle, either in the second wheel position or in the third wheel position, depending on what kinds of grinds the skater plans on doing. The remaining axle has an anti-rocker wheel, a special grind block, or is just left empty.
The purpose of a tri-rocker setup is to give the skater good speed and maneuverability, like on a flat setup. But because of the extra space, grinding is also a little easier than a flat setup.
Read more about this topic: Inline Skate Wheel Setups