wiring porcelain lampholder w/ grounded outlet

i am changing my plastic lampholder in my garage to a porcelain lampholder with a grounded outlet. I intend to use the outlet for my shoplight. There are three sets of wires runing to the outlet box. I wired the white ("neutral") wire to the silver screw and the red wire (which I believe to bet the "hot" wire) to the brass screw on the lampholder. The question is which bare ground wire do I use to ground the outlet on the lampholder? There are 3 bare wires that where twisted together with a wire nut. Do I use one of the exsisting 3 or do I attach a fourth wire to the lampholder and twist it along with the other 3 in a wire nut?

 

Yes. All grounds are always tied together and to the box also if it is metal. No wires should have been touched except the ones to the original lamp holder. The new lamp holder would go to the same wires.

 

Thank you for the information.