Running 3 Wire when you Only Need 2

Let's say for example I'm running wire to a receptacle and all I have on *hand* is 3 wire. If one was to use that, is it code-okay to just Cut the 3rd Wire flush to where the outer jacket is removed on both ends. Or does the 3rd wire need to be in the box with a wirenut. I don't do this often....but it just so happens that I need to run a spare outlet and only have 3 wire in stock. ///// And or just buy the 2 wire , which is a option

You can use the three wire in place of the two wire cable. Don't cut off the third wire. Just leave it long, cap it with a wire nut, and tuck it into the back of the box.

"Someday" you may wish you had a third wire in the box, so DO NOT cut it off. Just put wire nuts on both ends so it cannot accidently be energized by contact with the live wire on the outlet and shove it into the box.

The handy thing you could do with the third wire is to split the receptacle and power each half separately. This can come in really handy in some situations. You need a 240vac ganged CB so if one side trips, the other does since it would use the neutral as common for both legs (the current should cancel out, so that is why it doesn't have to be bigger in gauge).

Is this AC or DC Wire ?

AC.

I'm going to just pickup some 2 Wire. Box fill is a concern, hence I did not want to have the 3rd lead in the box with a nut on it..