I have 3 sets of wires going to an outlet. I would like to add another outlet from this one, how do i do it?
Be aware that all electrical boxes are restricted on the numbers of wires they are allowed to hold based on their basic volume.  (Over-crowding in the box is considered inherently unsafe.) That being said, for the new outlet, simply add the new cable into the box. Tie all the blacks together under one wire nut. And under this nut as well include one free length of black wire the same gauge about 10" long.  Ensure the wire nut is tight and all the conductors are captured. Push this wire nut to the back of the box.  Do the same with the whites and grounds. Reinstall the original outlet.