My house is.. "long". Can I place the main unit in a spot (the only spot with ethernet access) and then the 2nd and 3rd ones in a line extending out in one direction? In other words, the 3rd one will be pretty far from the main unit, is that OK as long as it's a certain distance from the 2nd unit? Or do they all need to be in proximity to the main? Thanks
Yes, you certainly can, it's one of the features of a mesh network. Just keep in mind that if you have them 'daisy chained' like that, you won't get the same wifi performance as you would if you had them in a 'hub and spoke' type arrangement with the router in the middle, as the 'backhaul' from the furthest mesh point back to the router takes a bit longer. Depending on what you're doing you likely won't even notice, it's a matter of milliseconds.
That being said, you might want to consider if you can hold off a bit, rumours have begun regarding new Google/Nest Wifi that supports Wifi6 (e.g. https://www.androidpolice.com/google-working-on-new-nest-wifi-router-wifi-6/). The current gen only support wifi 5 and I understand it's hardware-based, so a future software upgrade won't help. Wifi 6 should be significantly faster with much further reach than the current generation.