3 Conductor Phone Jack Uses:

Headphone Jacks - Left signal, right signal & a common ground is all you need.

Insert Jack - Found on mixing boards & elsewhere. Typically Tip = Send, Ring (the bit in the middle) is return. Portion near the plug handle is ground.

Balanced T/R/S Connections - T/R/S stands for "Tip, Ring, Sleeve", so as to distinguish it from it's 2 conductor brother. Typically tip = in phase signal, ring = out of phase signal & ground is portion near the plug handle. This one can get a little tricky as some manufacturers claim pseudo balanced, which doesn't really exist. It's not able to reject interference like a balanced connection. I've seen these listed as balanced in the brochure & unbalanced in the owner's manual. If you plug a regular 2 conductor into this you'll still get a signal because the out of phase signal will be shorted to ground by the longer ground section on the plug, but it will not be balanced, so it won't reject interference over long distances. One drawback to using 1/4" T/R/S for balanced audio is you can't lift ground easily, short of cutting the wire. With an XLR connection you can just insert an adapter.

Guitar Pedals - Manufacturers can wire the input so the longer ground section of a 2 conductor 1/4" plug from your guitar shorts out the 3rd conductor & turns the pedal on only when you plug it in.

2 Conductor Phone Jacks

Guitar/Bass/Keyboard outputs - All the ones I've seen are unbalanced 1/4" connections.

Mixing Board Line inputs - Typically unbalanced unless it's specifically designated Balanced.

Signal Processing - Unbalanced 1/4" connections are suitable to the short cable lengths involved in hooking these up.

Speaker Outputs

Except for speaker outs, almost always a instrument or line level signal, you just never know which one. -10 level is the one found in the home stereo world. +4 is from the pro audio world. Instrument is between -10 & Mic level.

Rarely used for a mic level signal. If you see one...run!