Soldering a 2.5mm female stereo jack
Simplest solution is to get a multimeter and start poking around.  

The three terminals are called tip, ring, and sleeve - tip on the end, ring in the middle, and sleeve closest to the connector.  

First, you want to check the link cable to see which pins are connected to which.  Get a multimeter and measure the resistance between the contacts at one end and the contacts at the other end, one pair at a time.  You want to see if the cable is 1:1 (tip to tip, ring to ring, etc.) or if anything is swapped around.  Then put the unknown connector on one end of the cable and use the multimeter again to figure out which of those metal tabs corresponds to which of the terminals.  

I'll give you a freebie: that tab you marked B likely connects to the sleeve.  You'll have to check the other two yourself.