Term used to describe turning something on and off at the mains. Especially useful if you are an engineer and want to 'big up' your work when you have fixed a computer system or router etc.
Differs from a Two pin reset which is to turn a battery operated object on and off (like a mobile phone)
I've performed a three pin reset on your 'wireless router' sir, which seems to have cured the problem.
Your friend rings up with a problem on his PC and you recommend they do a 3 pin reset.