Comet calls these "triplexers", but they are also referred to as combiners or splitters. They are basically band-pass filters, simultaneously passing RF in both directions through the common connector while the band-pass filters provide a high degree of isolation between ports.
* TX and RX from three radios at the same time into a single triband antenna
* Combine a triband radio with separate outputs into a single triband antenna
* Allow a single coax run into three antennas at the top of the tower, and three radios at the bottom, by using a triplexer at both ends of the coax run, avoiding cost and weight
Choose from versions with a variety of band-pass frequency ranges, with or without coax leads, and connector combinations.