@SantiMaster.22 

What does Signal Source show for 3rd floor Deco, not just where it connects but quality of connection. Example of good connection, two blue bars:

What is the WiFi speed that you are getting on devices connected to that Deco?

What is the speed of your Internet connection?

  @Alexandre. 

It appears as if it is connected to the Main deco based on the signal source which is the one further away. the signal varies between 2 and 1 bar but is mostly at the one bar status. The WiFi speed I have on the main Deco is 250 Mb/s but on the one on the third floor, I'm only getting 40 Mb/s. 

  @SantiMaster.22 

You will have to wait for Deco X20 firmware that will allow to choose which Deco to connect to. It is not available yet, as far as I can tell. Even then, note that each "hop" cuts WiFi speed by half. If you are getting 250Mbps from Main Deco, you must be getting around 120Mbps on second floor. Is this right? If it is, then when 3rd floor Deco connects to 2nd floor Deco, it'll deliver 60Mbps at most.

Until firmware is available, your only options are to move 3rd floor Deco further away from Main Deco, or to create additional WiFi interference between 3rd floor Deco and Main Deco. Example: if your 3rd floor Deco is just above Main Deco, place 3rd floor Deco on metal tray or multiple layers of household aluminum foil.

When signal between 3rd floor Deco and Main Deco becomes extremely weak, 3rd floor Deco will reconnect to closer Deco from the second floor.

  @Alexandre. 

Okay, waiting is the answer then. I'm going to give the aluminum foil a try if it works it would definitely solve many issues. 

And yes on the second floor I'm getting around 120. but that extra 20 on the third floor would be amazing.

Anyway, thanks for the reply.