Hello @r3c4ll
In addition to the advice from @olavrb I just wanted to add that it sounds like you're planning to connect three Nest WiFi Routers to an existing "outer" network and set them up with the same SSID as each other. In that case, each one will be limited to 5 Nest WiFi Point units. However, I would advise against doing this for several reasons. First, they will all still be sharing a single 5GHz channel with each other. So, significant traffic on such a large network may cause performance and reliability issues.
Second, even though they all have the same SSID, these three networks won't be integrated with each other in a way that allows for smooth hand-off between access points and routers from each of them. This may result in clients getting "stuck" to a router or access point in one even after moving fairly far away. For example, they may end up dropping from 5GHz (which has relatively short range) to 2.4GHz just to stay on the network they were on already.
Edit: Third, as an extension to the above, since each Router unit will also be a firewall that creates a new "inner" network, clients connected to each network will not be able to see or connect to clients connected to any of the other networks.
Lastly, such a large network is pretty far outside the intended and supported usage for this product. I would investigate alternative products intended for larger scale deployments such as this. I would also invest in running Ethernet cabling to support such a deployment.
