@MatiasRapa 

You can try moving 3rd Deco as far away from Main Deco as possible. If you have one floor apartment, that is the only option you have. When link between 3rd Deco and Main Deco becomes extremely weak, 3rd Deco should connect to 2nd Deco. TP-Link explained that connection logic in the following document: Is the Signal Source for Satellite Decos Selectable on the Deco App?

On a chance there might be a beta firmware for X50 with manual signal selection, send PM directly to @David-TP , this is TP-Link Support account that shares firmware beta for different Deco models. You only need to install beta firmware on 3rd Deco.

If you are happy with 200Mbps wireless speed, you can return X50s and replace them with 3-pack Deco M9 Plus. If your ISP Internet speed is over 200Mbps, Deco M9 Plus may deliver faster speeds than 200Mbps, especially around 2nd and 3rd Deco nodes. If your ISP speed is 200Mbps, M9 Plus can't be worse than X50. By the price, M9 Plus should be same or less expensive than X50.

TP-Link already offers beta firmware for M9 Plus, with manual signal selection, see forum topic Deco M9 Plus 1.6.0. Just like with X50, you will only need to install beta firmware on 3rd Deco. Because TP-Link shares M9 Plus beta firmware with users, the official firmware release must be coming soon.

In case you decided to return Deco X50s set and look for different WiFi mesh brand, check first with their Support or user forums if their WiFi mesh allows manual signal selection. Not all brands do, even not all major brands.