Three Way Valve Principle

three way
tripartite: involving three parties or elements; "a tripartite treaty"; "a tripartite division"; "a three-way playoff"
Involving three directions, processes, or participants
Three Way is a 2004 film about a kidnapping plot, based on the pulp novel Wild To Possess by Gil Brewer, the film stars Dominic Purcell, Joy Bryant, Ali Larter, Al Israel, Dwight Yoakam and Gina Gershon. The film was released also with titles 3-way and Three Way Split.
A threesome is a form of group sex involving three people of any gender combination.

principle
A fundamental truth or proposition that serves as the foundation for a system of belief or behavior or for a chain of reasoning
a basic generalization that is accepted as true and that can be used as a basis for reasoning or conduct; "their principles of composition characterized all their works"
a basic truth or law or assumption; "the principles of democracy"
a rule or standard especially of good behavior; "a man of principle"; "he will not violate his principles"
A rule or belief governing one's personal behavior
Morally correct behavior and attitudes

valve
A cylindrical mechanism in a brass instrument that, when depressed or turned, admits air into different sections of tubing and so extends the range of available notes
device in a brass wind instrument for varying the length of the air column to alter the pitch of a tone
control consisting of a mechanical device for controlling the flow of a fluid
A membranous fold in a hollow organ or tubular structure, such as a blood vessel or the digestive tract, that maintains the flow of the contents in one direction by closing in response to any pressure from reverse flow
a structure in a hollow organ (like the heart) with a flap to insure one-way flow of fluid through it
A device for controlling the passage of fluid through a pipe or duct, esp. an automatic device allowing movement in one direction only