The most frequent questions I have seen are related to 3-Way configurations. 

So here is a Tutorial that I hope will answer most questions, especially when it comes to installation. 

   What is a 3-way (or 4-way!) application? 

It is a wiring configuration to control a light from multiple switches in different locations - top and bottom of stairs is a common example.

Each switch will 'flip' the state of the light, regardless of what position the others are in. 

The Stair Lighting example mentioned typically has two switches - but it is even possible to have 3, 4 or even more switches all controlling the same fixture (for example a long hallway, where you might have a switch located outside each bedroom door) 

So, if lights are off, then switching the state of ANY of the switches will turn it on; if you leave that switch and go to any other in the chain, and flip that, it will turn light off again. 

Here is a graphic example of a three-switch configuration (the middle switch in the chain is a 4-way switch)
Note that this diagram is only showing the 'Line' voltage chain, which is what is being switched between the Mains Input to the Light Fixture. 

Those are the 8 possible configurations of the three switches - the first and last switches are 3-way types and the middle one is a 4-way (and if there more switch locations required, those would also 4-way)

Each switch - whether 3-way or 4-way has only 2 states. 

What is a 3-way Switch? 

A 3-way switch can also be called a 'change-over switch' - Technical Designation for it is a Single Pole, Double Throw (or SPDT) Switch

That means it has a common input and that will be routed to one OR other of the output terminals - but not both!!

The input is attached to the 'Common' (Line/Load designation on an HS210) and the Outputs are Traveler 1 and Traveler 2

So unlike a typical Single Switch, there is no 'Off' per se - the 3-way Switch redirects the Line Voltage to one or other terminal. 

Here's the simple part - an HS210 in regards to common switching, is no different from a dumb switch that it is replacing - it does exactly the same thing, switching the input terminal (Line/Load) to one OR other output (the Travelers), whether that be a manual operation of the Switch Paddle, or from your Smart Home application. Again, remember that the switch itself is never truly 'Off' - Note that the HS210 Switch is momentary action which electronically triggers an internal relay that flips the output state to the opposite Traveler, with each press of the switch.

This I think part is confusing to some - let's say you set the HS210 to a state that turns the light on - if you go to one of the dumb switches and flip that, it does not change the current state of the HS210.  The light goes out because the switch you operated flipped state, NOT the Smart Switch. So the flow diagram above applies regardless of whether it is Smart Switch or a Dumb switch in the first position. 

The Indicator Light itself is very clever (Smart even!!!) - it does not truly indicate which output is selected; it indicates when no current flows all the way to the load, regardless of which output is live. So the LED being on, tells nothing about whether the Relay is switched to the T1 or T2 terminals, only that (whichever state it is switched to) is flowing no current. i.e. it is NOT an on or off of the switch, it is reflecting an on or off of the LIGHT!. 

What are Traveler wires?

These are installed in the wall cavity BETWEEN the switch box locations. Unlike the incoming Line Romex and the Load (light) Romex which have Line, Neutral and Ground, the Traveler Romex is going to be a 3-wire plus ground - this would be Black and Red for the two Line Travelers, plus Neutral and Ground. When the circuit has been powered there will always be (only) ONE of the two Travelers in any given Romex, Hot (or live) 

What is a 'Smart' Switch? 

A smart Switch is simply one that can be controlled remotely, in this case of the HS210 via a WiFi command from your Local Network, from your phone, tablet or Alexa etc. 

A 'Dumb' switch (I often see these erroneously called 'Dummy' - it's Dumb', as in 'not Smart') is a purely manual physical switch which must be actuated directly 

Installation. 

This question gets asked over and over - how many HS210 do I need for my 3, 4, 5 switch configuration? In ALL cases the answer is 'Just One' 

As explained above the HS210 is essentially the same as a 'dumb' switch in its electrical config, the only difference being that it ALSO requires a connection to the Neutral (the line AND neutral is required for the power supply inside the module, which controls the electronics and wifi). Some older homes light switch boxes may not include a Neutral - if that is the case, unfortunately you cannot use the HS210 (or any of the Kasa Switches) as it is absolutely required. 

Now - again, we only need one HS210 - however it is important that it goes at the LINE end of the chain (the switch box that has the incoming power from the Breaker or Fuse Box). We need to identify the Line Input box, so we can ensure the HS210 is always powered, regardless of the State of the switch. 

It can only be one of two - only the first and last in the chain will be 3-way switches; if you pull the switch and find it is a 4-way, that immediately tells you it's not that one! 

So you have to idnetify which is the Line Box and which is the Load Box - they will both contain a 2-way (plus ground) and 3-way (plus ground), for the pertinent switch (of course there may be other cables in the box if it is a 2-gang or 3-gang installation). In order to validate that you will require a Voltage Tester - the simplest and safest is a 'Non Contact Voltage Tester' - do a search on Amazon or find at your local HD or L Store. These work as their name implies - it does not have to physically contact an exposed wire or terminal, it will even read through the wire insulation. 

First, guess at which it might be - you have a 50-50 shot 

Turn off the breaker, check with your tester that the terminals are not Live, and remove just that one wire that would be either line or load - that wire will be attached to the BLACK terminal on the Dumb Switch. Now put a spare wire nut on that exposed copper conductor. Again, at this stage do not remove any other wires. Now turn your breaker back on and test that wire with your non-contact tester - you will get an indication if you have the right one. If it's dead, then that would be the Load Wire - you guessed wrong! 

Turn off breaker, reconnect that wire to the same Black Terminal and re-install that switch back in the box - there is nothing needs to be done with that one.  

Repeat the process for the second box - since the other was the load, this one HAS to be the Line - get a confirmation then turn the breaker back off. 

At this stage, you can now remove the dumb switch from that box completely 

Connect up the HS210 with the newly-identified Line to Line/Load, the travelers to those two terminals (it does not matter which is which!!!) and connect your ground and neutral. 

Here is the schematic that shows how it will be connected (just the Line and Load boxes are shown - if there are more boxes/switches they are moot as far as the Smart installation goes) 

That should complete your wiring, you can power on the breaker and proceed to the Wifi Set-up per the App.