Have a 3 way switch at the top of stairs that is wired to one light. The rest of the wire was dropped thought the floor and I was told that the white wite was the common. This is 12\3 wire red black white and ground . I ran the 12\3 that was thought the floor to a 3way switch and from there to another other lights with 12\2 I just need to know how to wire the switch I put in to make the lights turn on and off with the light at the top of the stairs .
Thanks Brent

Verify the two traveler screws on the first switch are hooked up with the red and black from the 12/3. This will verify that white goes to the common screw on the switch down stairs. The red and the black wires go on the traveler screws on the 3 way switch down stairs.

Mark the white wire with a wrap of black tape so it doesn't get confused with a neutral on the ends in either box.

Assuming the power source is split in the upstairs switch box, there will be neutrals from the wire to the light and from the power source, both white, that you tie together. Next, the white wire from the 12/3 in the upstairs box will get connected to the black going to the light. From there, if you wanted to run other lights, you could either run them from the box where the light connects or drop a new line from the switch box upstairs connecting the white to the other neutrals and the black to the common (white) returning the current through the switch downstairs.

Your post read to me as if you wired in lights of the switch down stairs. Unless you ran a second 12/2 from upstairs, you have no neutral down stairs of the common so a pigtail from that won't work.

If the power is in the light fixtures box, then it was sent to the first 3 way as a switch loop. The white wire feeding the box when tested with a light tester jumping ground should light up. If not, check the black. Whichever one lights up is the hot from the ceiling box or wall sconce. This is the wire that would be connected to the "common" screw in the top switch. The other wire from the pair gets connected to the white (common) of the 12/3. With this configuration, you have no neutrals in either switch box and can't take from either switch box for an additional lighting run.

That can be fixed by replacing the 12/2 coming from the light to the switch... because in this case, the hot is broken at the light, routed over one line of the 12/2 to the switch and returned on the other line of the 12/2, making the white from that one into a hot. To get a neutral into the switch box for other runs, you could replace the switch loop leg of 12/2 with 12/3 and use the black and red for the "over from" and "back to" the light legs of the run and the white in that length of wire can be capped together with the neutrals in the light box. From there you could run 12/2 to wherever other lights or you could have not done all that and just ran the lights from the other light instead.