I am installing a 3 prong outlet for my welder, using 8-2 wire. The store gave me 8-3 wire and said to leave one wire out, so witch one do I leave out? I have red,black, white, and a bare ground.

Thanks

Assuming you need straight 240 volts with no neutral, cap the white wire. This will leave the black and red as the two hots, and bare as the equipment ground

leave out the white. Red = hot, black = hot, and bare = ground.

LOL!
Good answers!