When it comes to components used in a cold runner system's mold, the only difference between a three-plate mold and a two-plate mold is that a runner plate is added to the three plate mold's configuration. These molds consist of two parting planes, and mold splits into three sections.
One of the key advantages to these designs is that the runner is ejected separately from the molded part. Which means it will not need to be manually cut off. On top of that, three-plate molds give engineers more gating location flexibility, and they're a lower-cost alternative to hot runner systems (when applicable).
Unfortunately, three-plate molds have a longer cycle time, more complex tool design, require a high injection pressure, and typically results in higher material waste.