Pin 1 is "positive" if your system is setup like that. THERE IS NO REASON IT COULDN'T BE NEGATIVE! "Positive Tip" is something of a common standard, but there are examples where this is not true (for example - guitar pedals pretty much are all negative-tip, positive shield)
Pin #3 is a switch that is 'normal closed'. When no plug is in the jack, pins #2 and #3 are shorted together. When you insert a plug, pin #3 actually moves and becomes disconnected from pin #2. In this state, there is no electrical connection between pin #3 and the plug.
This is useful when the device has both batteries and wall-power input. When you insert the plug, it automatically disconnects the batteries (thus preventing your wall-power from charging the batteries)
Most likely, in your case, you would make pin #1 "positive", pin #2 "negative" and pin 3 not connected. But only you can know if this is right or not as we don't have access to the rest of your circuit or understanding of what you're trying to accomplish.