Originally Posted by Stubbie
Just rereading I see the 14/3 is getting power to the upstairs switchbox so you cannot use it as KBsparky said You will have to get a new cable to the upstairs switchbox.
No so fast, Stubbie. The OP stated it was not feasible to feed another wire to that switchbox. OK, I accept that.
What you missed was I suggested that the rest of the circuit might be back-fed from anywhere else
-- an outlet, light, or switch -- doesn't matter where, as long as its the same circuit. If the basement is open and unfinished, then that might be an easier task than trying to get another line up to that 3-gang box upstairs.
IF one can back-feed the rest of the circuit, then the existing 3-wire cable is no longer needed to act as the feed for it, thus allowing one to reassign those conductors as travelers in a properly wired 3-way switching scenario.