Three big wheels offer a bigger footprint with more PU in contact with the ground and more response out of the corners. So that's why less wheels mean more grip. Also three wheel frames can be lower and shorter than traditional 4 wheel frames with smaller wheels, making it easy to maneuver and control your stride, and with the middle wheel right under the center of gravity, turns are easier and the whole skating experience feels more natural.
All these features make it easier to adopt a proper skating habit and help you learn advanced techniques such as the double push. An additional benefit is the less weight which makes inline skating more fun and less energy consuming, allowing to skate for a longer time and improving your endurance.