Kazoo is correct.
Where I live, you can use an outletl box as a junction box provided it is deeper than usual.
Without pulling out my electrical code, the Ontario code states that a standard 2.5" deep box is allowed 5 conductors total (ie. 5 No. 14 wires (not including grounds)
The reason is heat transfer. The number of wires allowed inside the box is a function of the volume of the box.
If you want to go to 6 conductors, you need to use a deeper box (which has a larger volume), like a 3" box.
Again, this is the Ontario code.
One other thing you have to watch is that it is not a split outlet.
Is the outlet in the kitchen by any chance?????
Since you say there are 3 neutrals and 3 hots, it is unlikely it is a split, but check to make sure anyway. Stranger things have happened.
Make sure that the small copper tab between the two screws on the side of the outlet have not been clipped. Kitchen outlets are often "split circuits" in order to allow 15 amps to the bottom plug and another 15 amps to the top plug (using a different circuit than the lower recepiticle on the opposite phase so as not to over load the neutral)
http://www.askthebuilder.com/452_Bew..._Repairs.shtml