What is a 3-pole cable?
I may be wrong, but to me it looks like 3-pole plug refers to the usual Tip Ring Sleeve 6.35mm, 3.5 mm or 2.5 mm plug. There are also variations such as tip sleeve and tip ring ring sleeve and similar plugs. 

Here's a picture from the Wikipedia of such plugs:



Going from left to right, we have a 2.5 mm tip sleeve connector, 3,5 mm tip sleeve connector, 3.5 mm tip ring sleeve connector and 6.35 mm tip ring sleeve connector. 

Usually, the tip ring sleeve 3.5 mm is the default plug for consumer stereo audio, so you'd need a matching 2.5 mm plug. 

The poles part probably comes from internal connections of the connectors. Usually each section (tip, rings and sleeve) are electrically isolated from each other and each can carry a single signal.