tookwik

In my kit i recieved 2 prewired male end prongs to plug to the existing socket, but... there is a 3rd hole for a 3rd prong. The kit included a wire with a 3rd prong and a splitter which are not connected to the other 2 wires with prongs. My question is what am i suppose to connect the end opposite the prong to on the 3rd wire? I'm pretty sure i should run it into the splitter, but what wire should i connect it to? red or black or (+) or (-)? Any suggestions would help.