Hello @Kalle_123
The nodes in the mesh will use the shortest path to get their traffic to its destination. So, if your point 3 can get through to the main point directly, it will do so – even if it's a relatively weak connection. Going through point 2 might or might not be faster, since any traffic point 3 sends to point 2 will have to go back out over the same channel again to get to the main point. This is why the placement advice recommends trying to put the main point as close to the center of your home as possible with mesh secondary points placed one or two rooms away from there where they can provide coverage to more distant clients.
As for your second question, this gets somewhat esoteric. The short answer is that connecting point 2 to point 3 via Ethernet won't work reliably. The longer answer for why that's the case has to do with how the mesh protocol works. It is only running over the 5GHz WiFi interface – it doesn't run over Ethernet. So, when you connect two nodes via Ethernet that are already connected to each other via the mesh network, it creates a traffic loop. To break that loop, Google WiFi uses the Spanning Tree Protocol to detect the redundant path and disable one of the interfaces on one of the two nodes involved in the loop. The interface it disables will be the mesh interface, which means that node can no longer communicate with the rest of the mesh. If that happens on point 3, it might work. But, if it happens on point 2, then bad things will happen, since all of point 2's client traffic will go over Ethernet to point 3 and from there over the (weak) mesh connection to the main point. That would not be ideal. Unfortunately, since STP is a peer-to-peer protocol, we can't force it to pick point 3 rather than point 2 – it will essentially be random, and it might be different every time it powers up.
I hope some of these details help. The best advice I can give is to run Ethernet from the main point to at least point 3, and preferably point 2 as well. That will provide the best possible performance without changing the placement.
