I have a fixture in the basement controlled by two two switches, one at the top of the stairs, one down in the basement. I want to add a light to the top of the stairs and have it controlled by the same switch. I ran a wire through the wall to one of the switches, but I'm not sure how to connect the fixture in to the circuit. The fixture I'm adding has the standard black, white, and ground lines. The switch has the black, white, and a red line. The line I ran has black, white, red, and ground lines. Any suggestions?

You would have been better of extending the new fixture from the existing fixture.

You may not be able to connect the new fixture to just any 3 way switch. That switch must have the switched leg and a neutral in the switch box, if not, then no go.

3 ways switches can be wired in multiple ways, and there is no standard as to which switch has the switched leg and neutral. It all depends on who installed it and how they arranged the wiring, there is no right or wrong way.

Sounds like if you only have a black, red, and white in the 3 way switch box, you do not have what is needed the to control the new light.

You will not have any problem if you run the new cable to the existing light.

In addition to above. One three way will have a constant hot on the dark(common screw),
The other three way will have the switched leg(goes to light)on that 3 way dark screw.
Likely that on is down stairs, since light is down stairs. If you get lucky and notice the switched leg 3 way IS upstairs, and it has a white(neutral), then just connect to that switch
Leg, and neutral to new light. Just measure voltage on dark screw, if it stays hot when switching, then that is "Hot End", other would be "switched end".
If the white is on the switch, it is not neutral.