I'm trying to install a dimmer switch on a 3-way switch (there's another switch that also controls this light). The old switch has a black and white wire each connected to a brass screw, another black wire going into a hole labeled common, and a third black wire attached to the black screw next to the common hole. The bare ground wire is coiled up in the back of the box. The directions only mention connecting 4 wires, including the ground. We initially got the light to work and dim when connecting the third black wire to the ground on the new switch. However, another light and ceiling fan didn't work. I suspect one of the black wires goes on to power the other light switch and fan downstream. Where do I attach this wire to the dimmer?
You'll have to take the black wire that was hooked to the black common screw and hook it to the black wire that went into the hole marked "common". Then both of those wires get connected to the "line or hot" wire for the dimmer. The other two wires get hooked to the dimmer per the instructions as does your ground wire.