Wiring for a regular switch to a light
2 wires into wall box
2 wires out of wall box
Black - Hot wires
Grey - White neutral wires
Ground - not shown, assume it's in there
Wiring for a 3 way switch to a light
2 wires into wall box
2 wires out of wall box
Black - Hot wires
Greg - White neutral wires
Ground - not shown, assume it's in there
Wiring for a 3 way switch to a light
2 wires into first box, 5 wires out
5 wires into second box, 2 wires out
Black - Hot wires
Greg - White neutral wires
Red - Traveler wires
Ground - not shown, assume it's in there
I'd have to run an extra cable to get this to work. Could get it to work in a new build.
Connect the Shelly 1 as though you were connecting a regular switch
Pass through the wires in the second wall box, cap off wires that are not needed
Make a switch from an ESP32/8266 and put it close to where the second switch was
Shelly 1 can be moved to the other wall box if needed
Control the Shelly 1 using MQTT / Home Assistant