Wiktionary
three green
phr. (context aviation English) a confirmation given by the pilots that the landing gear has gone down successfully and has locked.
Usage examples of "three green".
He would look up, and before him he would see, astride their foaming wild asses, the three green horsemen motionless, pointing at him with their long forefingers.
The rhythm of the three green horsemen that was to have been the prelude to the Queen of Sheba began rollicking through his head.
He walked down the three green steps, and went straight toward the mob, to the man holding the torch.