We are also in a three-storey EFH. The router is on the middle floor and I bought the Deco X60 on the recommendation of a neighbour. I'm not an IT expert, but the connection to the first and second floors works perfectly and the speed doesn't drop below what the router delivers.

I moved from Orbi's to these, as Orbi's kept cutting out at LEAST once a week. You won;t gain any speed with them, but you will gain peace of mind and not likely to throw them at the window, which I wanted to do verey week with the Orbi,