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The quarter-inch 3-pole jack is a very versatile connector. It doesn't have all the 'goodness' features of an XLR, but it's compact and mostly reliable.
The three poles are the three separate electrical connections it allows, named the tip, ring and sleeve.
The principal use of the 3-pole jack is as a stereo connector. You will often find a 3-pole jack on a pair of headphones, and if you don't find the full-size jack connector, you'll find a miniaturized one (that probably came with an adaptor).
Since a single signal connection requires a minimum of two conductors, you might wonder how in this case 2 x 2 = 3. That's because the earths of the two signals can be joined together. They are both at zero volts so it makes hardly any difference.
But that's not all a 3-pole jack can do. It can also be used as an 'insert' connector on a mixing console.
Most mixing consoles have 'insert points' in the channel strip, usually after the preamp and before the EQ. Sometimes they come after the EQ. The idea is to connect a compressor, gate or external EQ to the insert point to process the signal. Once you have this facility, you'll find you can't live without it.
The insert connector on all but the most expensive consoles will be a 3-pole jack socket. It isn't wired for stereo though. The tip is the 'send' of the insert, which sends signal to the external processor. The ring is the 'return', which accepts the processed signal back into the console. Confusingly, some consoles are wired the other way round.
When there is no jack connected to the insert, internal switching automatically bridges the signal across.
To take advantage of your insert points, you need to buy or make up special insert cables. You can recognize these because the 3-pole jack at one end sprouts two cables, leading to two individual connectors, which may be jack, phono or XLR according to your needs. An insert cable looks identical to a splitter cable from the outside, which it most definitely is not, so it needs careful labeling.
One further use for a 3-pole jack? Wire the ring and screen together and use it as a mono 2-pole jack!