Seems to be used for making differential gearboxes, cars, and GBCs.
Here you have the Bricklink link to the part: https://www.bricklink.com/v2/catalog/catalogitem.page?P=63869#T=C
it is named Technic, Axle and Pin Connector Perpendicular Triple on BrickLink. The ldraw/bricklin item id is 63869. The Lego design id is either 63869 or 42191, indicating that there have been at least two moulds.