The third pipe is called a "process stub", as I've heard it.
When they make the fridge, etc. They hook a vacuum pump onto
the process stub, and evacuate the air. Then, they fill with
a charge weight of refrigerant, and oil. The process stub is
pinched off, and brazed shut. If nothing is leaking, the
refrigerator, or window / wall AC is good to go for many
years. There is a special tool called a "pinch off tool"
which is used for this.
Christopher A. Young
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