Spacer had gone back to the basics and threw out the notion that a fountain pen nib had to resemble the old quill and dip pen with single slit between two tines. Just as three legs are sufficient to make a table stand on its own, so Spacer discovered that three sets of tines would allow a pen to write at any rotation and angle. If the three sets of tines were touching, the point of contact between these sets would also serve as slits to retain ink, thus giving the pen six slits in total. The combination of all these tines coming together would make the tip strong enough to bear up under the pressure required to make carbon copies.
Found via a discussion on The Fountain Pen Network.