Designated Third Wheel

A designated third wheel is a special type of third wheel.

S/he is supposed to barge in and purposefully interrupt a steamy love session because the couple just can't pull apart once they get at it.

Used to keep track of time and bring them back to "the real world".

Antoine: "Man, things got steamy with Veronica the other night. We kept at it for seven hours! Her parents were furious."
George: "Damn bro, you need me to be your designated third wheel?"