Conveyor Merger

A Conveyor Merger is a building that takes items from three input belts in sequence and outputs all items onto a fourth belt. If any input is missing the merger takes the items from remaining input belts. If the output belt is slower than the total speed of all input belts, the input items have to wait.

Trivia
It is easy to differentiate between input slot and output slot by looking at the shape of LED light at the belt connector. The output is indicated by a arrow-like light, while the input is 3 smaller dotted light.