Wirelessly extending a Wi-Fi signal always results in some 50 percent signal loss because the extender has to do two jobs at once: receiving the Wi-Fi signal from the original router and rebroadcasting.
That said, if you use two units of the Google Wifi, devices connect to the satellite unit will need twice the amount of time compared with those connected to the main router unit to receive the same amount of data. And if you use three units, this could get even worse.
If you just want to use the internet, the signal loss won't matter much since Wi-Fi is so much faster than most residential broadband connections (
if your internet speed is faster than 200Mbps, and you want to use that at full speed, you definitely don't want to use the Google Wifi or any wireless Wi-Fi systems). However, if you want to do heavy local tasks, such as backing up your Mac to a Time Machine server, or transfer a large file from one computer to another, the Wifi isn't the best choice.