Hello,

Cause you have 3 floor to cover and several client devices  to be connected,  if you want to replace the current product, it is suggested to use the comcast modem with TP-Link whole-home wifi system product, which is called Deco. Like Deco M5, Deco M9 plus. 

https://www.tp-link.com/en/home-networking/deco/

They can connect to each other by wired cabld wireless; that is to say, you can locate each one at each floor to ensure the wifi coverage and signal strength. 

May it help.

Good day. 

@katychouli 

Cause you already have a wireless router, if the whole house is running under the same power circuit, you can use powerline adapters with wifi function to extend the wifi network.

https://www.tp-link.com/en/home-networking/powerline/?filterby=4893

Or you can buy a Deco series to replace the current router and relocate the Deco nodes to extend the wifi range. 

https://www.tp-link.com/deco/