Hello @Bbwacker
Concrete is a significant problem for WiFi, especially in the 5GHz band that the wireless mesh uses to interconnect. You can start adding those new units to your existing mesh (pretend the new Nest WiFi Router unit is just another Nest WiFi Point when adding it in the Google Home app), but I would not expect good performance through a concrete floor. If the interior walls are also concrete, it's just going to be even worse. Do you have coaxial cabling in place, by any chance? If so, I would buy some MoCa adapters and connect them to the primary Nest WiFi Router's LAN port and the new secondary Nest WiFi Router on another floor in hopes of bypassing the wireless mesh at least a bit. However, that wired secondary won't be providing mesh-only coverage to nearby Nest WiFi Point units, so you might be better off returning the new three-pack Nest WiFi system and buying a three-pack Google WiFi system, since those all support Ethernet and could be added to a MoCa backbone through adapters.
If you don't have coaxial cabling in place, I guess you could try using powerline adapters, but in my experience they don't work great. They might still work better than trying to build a wireless mesh in a concrete house.
I know you said running Ethernet isn't an option. But, if that's really the case, and the MoCa and powerline options don't work, then the alternative may be not having a reliable WiFi network in other parts of the house. I know this isn't the news you want to hear, but WiFi is just extremely low power, and it just can't get through solid materials like concrete.
