3 terminals Tip, Ring, Sleeve is also commonly known as a TRS. Heres a PDF link that shows a 1/4" TRS for reference. plug
http://www.neutrik.com/images/ock/pr.../210_12221.pdf
You can do one of two things depending on what kind of wire your using. If you are using single pair non-shielded wire just jump the tip and ring at the jack. The tip/ring would be the Hi and the sleeve would be Lo.
Single pair Shielded wire: Shield to sleeve, Red to tip, Black to Ring at the jack. At the input tie red and black together. Red/Black goes to Hi and the shield goes to Lo. This is how I usually interface iPods, computers & whatnot into large sound systems at work.
If you only hook up Tip (usually left) or Ring (usually right) you will still hear in both ears because the intercom is mono but if there is a guitar or something in the music that is panned hard to the wrong side you won't hear it.
hope this helps!
Jeff