What can cause 1 light to dim when 3 others in the room are ok the light works from its own 2 way switch?

The switch sounds like it might be arcing. Switches are inexpensive, change it out with a new one. With the power off and the old switch removed check the connections in the switch junction box for loose splices or burnt wire nut connectors. If everything is fine there check the light junction box for loose connections.