How can you have 17 volts on two # 6 wires that are not hooked up on all ends. The third wire is hooked up on both ends and is receiving 120volts. We tried twisting the wires and it made no difference. We tried uf and still voltage on the other wires that were not hooked up.

Its induced voltage from magnetic fields on unterminated wires... you would have to ground one end them to get rid of it. Or leave it alone. My vote is leave it alone... you might need them at some time in the future and won't remember where they are grounded.

As long as they truly unconnected, tagged and insulated for future use

Silly little questions

What are the colors of the conductors in the outlet box?

6 AWG wire can handle 55 amps, Did this receptacle used to belong to a clothes dryer?

You said you traced the third conductor and found it was connected at both ends, correct? If so, what is the conductor connected to on each end. At the panelboard are there any blanks next to the connected conductor?