Hello and welcome.The three radially spaced tabs act as a trigger for the locking mechanism of the connector, much like the pins and barrel of a lock.
If you rotate the connector into it's open/ receptive position, the tabs pop up and sit flush with the rim of the connector. Three sprung studs in the bottom half of the connector pop up and lock the valve it the open position.
On all male connectors there is a smooth parallel section of tube with a rim at the top. Insert the male tube and it is this rim that depresses these three tabs in the top half, which in turn push the three sprung locking studs clear of the top half into the bottom half of the connector and allow the top half to rotate into the closed position.
Hope that helps!