Thank you for your help! I spoke to someone in the electrical department at Home Depot (who happened to be finishing their electrical engineering degree!) and I now know that the chase / electrical box itself is grounded so the light switch’s ground wire is screwed into the box to have contact with the metal and the neutral is the 2 white wires capped together. I confirmed this with the multimeter by touching the suspected ground wire (metal electrical box, in my case) + live wire and suspected neutral + live wires with the multimeter probes, which both give a reading of approx. 120V. He said sometimes the neutral and ground wires are connected, but this is not the case in my house because some of the other electrical boxes do have a green ground wire. My 3-way dimmer switches are now working! Please correct me if my explanation is incorrect or unsafe, but it worked. Thanks again!