Originally Posted by
CitizenOfDreams
No, 3 rails would not be a good idea. Quote from a Nippon Bearing North America white paper:
Because two parallel single-rail guides spread apart is the proper approach to carry large loads, many engineers assume it’s acceptable to use more than two rails to carry even heavier loads. One reason they’re tempted to do this is to avoid the cost of a larger rail. While increasing rail size may be more expensive, using three or more rails and carriages produces a statically indeterminate system and can lead to rough operation. Unless installation is perfect, rails and carriages will inadvertently be slightly out of line. The expense of installing such a system accurately enough to produce smooth motion will likely exceed the cost of using a larger rail.