The 3 way switch has "common" pointing to one screw (I believe the black one, the other 2 screws are brass colored).Which wire is the "common? The 120 volt or the wire to the light or the wire between the 2 switches?

Black screw is the common and it gets your black wire. Read completely How to wire a 3 way switch

Hot is to the black terminal, traveler wires are either of the other two terminals. Wiring is different when power is at the switch or at the ceiling box. Remember, hot is always switched. Look at the link posted.