I need to put a GFCI in a 3 cable outlet
I am having a problem with an outlet. I have been replacing outlets in my old home with GFCI. I know how to do a GFCI outlet with 2 cables or 1 cable, but I have recently encountered a problem. I opened an outlet and it had 3 cables, each with two wires. All the blacks from each cable were wired into hot, and all the whites from each cable were wired into neutral. Each grouping of wires (one black group, one white group) were what I would call "spliced" together and attached onto the screws. They were attached onto both line and load for either side. I am a novice and I don't really understand what is going on here. I have attached a diagram of how the outlet was before I ripped it out. Some extra info which may be relevant: I think this outlet used to be used with a window air conditioner. It is the only outlet I have encountered so far with 3 cables and 6 wires total. None of the outlets in my home are grounded that I have found so far.
http://tinypic.com/r/fk8m8m/5