Can I use only one 3-way switch where 2 switches control the same light or should I need to install a pair of these?

You could replace both only if you can't figure out which one you need to replace. I have 4 switches that control my hallway lights and I replaced only 1 and it works perfectly. As others have said, you need to replace the one switch that sends feedback to the other switches the state of the light. I must have got lucky because I didn't know which one that was. Here's a hint though when I had to search for answers: 2 of the switches contained 5 wires: 2 black, 2 red, and 1 ground. The other 2 switches had 4 wires: 2 black, 1 red, and 1 ground. I replaced the switch with 4 wires because this smart switch had 4 wires to connect. I connected the common wire to the common port on the smart switch. Then I connected 1 red and 1 black write that came from the same "bundle wire" in the switch box to the traveler wires port on the smart switch and voila. App works, other dumb switches work perfectly. There's no having to leaving other dumb switches on or off. Turning on the light using the dumb switch sends feedback to the smart switch and it changes the state.

Will one switch work on a 4-way configuration (3 switches controlling 1 light)?

Tldr: I'm pretty sure you need to replace both 3 way switches in a 4 way circuit, at least if you want it to work as intended. Fwiw: I did not get a 4 way system to work with only one kasa 210. It kinda worked (could switch in some states, Alexa /kasa never knew of the light was on it not). But checking the tp link forum, users had success by switching out the 3 way switches. So, for my 4 way switch (3way then 4way then 3 way) I replaced BOTH 3 way switches with Kasas. Very important... Leave both switches in the setup state (blinking orange and green, delete from app if you already setup). Setup wifi on only 1 switch. Kasa app will ask if you replaced 2 switches in the circuit, say yes. Follow the directions very carefully (they are different than the usual wifi config, note that you are looking for orange/red blinking this time). If you do this correctly, you will ONLY HAVE 1 SWITCH LISTED IN THE APP! If you have both switches listed independently, you did it wrong and will need to start over (delete/reset, etc). Though only 1 switch shows up, they both work and the circuit can be controlled by voice.

Operation Mode	ON-OFF-ON
Current Rating	15 Amps
Operating Voltage	120 Volts (AC)
Contact Type	Normally Closed
Connector Type	Clamp
Brand	Kasa Smart
Terminal	Spst
Item Dimensions LxWxH	4.13 x 1.71 x 1.77 inches
Circuit Type	3-way
Mounting Type	Wall Mount
Actuator Type	Push Button
Contact Material	Plastic
International Protection Rating	IP00
Lower Temperature Rating	32 Degrees Fahrenheit
Upper Temperature Rating	104 Degrees Fahrenheit
Controller Type	Google Assistant, vera, Amazon Alexa
Control Method	Voice
Connectivity Protocol	Wi-Fi
Color	white
Wattage	600 watts
Unit Count	1.0 Count
Number of Items	1
Manufacturer	TP-Link
Part Number	HS210
Item Weight	5.3 ounces
Product Dimensions	4.13 x 1.71 x 1.77 inches
Country of Origin	Vietnam
Item model number	HS210
Style	Single Pack
Pattern	Switch
Shape	Rectangular
Voltage	110 Volts
Item Package Quantity	1
Special Features	Dimmable
Included Components	1 x Kasa Smart 3-way Switch
Batteries Required?	No
Warranty Description	2 year manufacturer

Kasa Smart 3 Way Switch HS210, Needs Neutral Wire, 2.4GHz Wi-Fi Light Switch works with Alexa and Google Home, UL Certified, No Hub Required , white

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