https://support.google.com/mail/accounts/answer/78754 (Failure)
The way that I have found to work 3 way switches with Sonoff is by using push buttons.
There are a red, black, and white wires going thru each boxes that can be used to toggle the sonoff, but running 3.3 volts to activate gpio14 will pick up lots of noise, and a lot of flashing lights. So I put a power source in one of the boxes, the voltage must only match the relay. In the case of the 4 way switches, splice the red to red, black to black, and white to white. Now connect the normally open push button from black to red. I the box with the sonoff install the relay with the coil from red to white, and the no contacts from gpio14 to ground. Also, connect the push button the same way from gpio14 to ground. I have done this with 110vac and a 110volt relay. Also, I have used a 3.8 volt battery I had laying around and a relay from a old, bad sonnoff I had. In order to make the sonoff work with a button, you need to connect to the sonoff, go to console, and enter “Switchmode1 4”.