@SynaZe 

In Deco app, open information page for 3rd Deco node. You should see its Signal Source, example:

In this example, signal strength is good, two blue bars. What does it show for your 3rd Deco signal strength?

Touch Signal Source text in information page for 3rd Deco: if Connection Preference screen does not open, your firmware does not have the ability to adjust connection manually. Not very many Deco models have it yet, but TP-Link plans to add that feature, eventually. See TP-Link explanation: Is the Signal Source for Satellite Decos Selectable on the Deco App?

It might still be possible to force 3rd Deco connect to the 2nd, by repositioning them or by other means, but that depends on specifics of Deco mesh deployment at your house. If you want to try that, I'll ask you additional questions to figure what could work for you.

  @Alexandre. 

Thank you for the reply! 

I tried to click the signal source text but nothing is happening.
The current setup is one near the modem at the ground floor, the second node is located in the kitchen on the ground floor.
The third node is located on the first floor in the living room.

Second node:

Third node: 

Which additional information do you need in order to help me?

  @SynaZe 

I will need to have floor plan of your house, both floors, with where all three Deco nodes are placed. It could be similar to what was shared in this conversation https://community.tp-link.com/en/home/forum/topic/535040, or something simpler, like this:

What I need to see is location of each Deco relative to other Decos, and on both floors. Please make sure you add Deco node names to the floor plans, so that I won't need to guess which Deco is which.

  @SynaZe 

Sorry, one more thing: what is the speed of your Internet connection? 

@Alexandre. 

I tried to work something out, don't look at the measurements they are probably way off.

What do you mean with current speed? You mean speed when I'm on the third deco or just the general internet connection speed?

  @SynaZe 

I am asking about general Internet connection speed, what you are getting from your ISP. If you can still return X50s and exchange them for different Deco model, I can recommend those that have Signal Source manual selection already. The problem is, these other Deco models are WiFi5 only, I would not recommend those for ISP speeds in excess of 500Mbps.

As for what you have, the options are: make 3rd node connection to Main Deco even weaker, so that it will be forced to switch to 2nd Deco OR make 3rd node connection to Main Deco stronger and eliminate weak Signal Source issue.

I've updated your floor plan with recommendations where to move 3rd Deco to see if new place fixes the issue. The end points are marked with stars, but it is the direction that is important, not the end point necessarily. Move 3rd Deco in these directions and see if you could find sweet spot for it, where it either connects to 2nd Deco or gets two blue bars for connection to Main Deco.

Another option is to leave 3rd Deco where it is, but artificially weaken its link to Main Deco even more, to the point where 3rd Deco is forced to connect to 2nd Deco. The link is already weak, so little push might do. I painted it in red color, but what it really means is to put few layers of household aliminium foil, or metal tray, or special WiFi blocking material (google that) under 3rd Deco and adjust that in such a way that it blocks direction from 3rd Deco to Main Deco as much as possible. 

For example, if it is a metal tray, place 3rd Deco on one corner of it with the rest of tray positioned in the direction of Main Deco.

  @Alexandre. 

Sorry for the late reply. My max should be download 1 gbps and upload 40 Mbps.
I will try to play around a bit with the location of the third node, would it also be a possibility to play around with the position of the second node?

Thank you anyway for your great advice/answer, really helpful!

  @SynaZe 

You can move second node, too. What they don't tell in marketing materials is WiFi mesh needs careful positioning of nodes to make it work well. This is applicable to any WiFi mesh brand, not just Deco.

Your objective is find such places for your Satellite Deco nodes where they can provide good WiFi coverage for all your house and report good WiFi link to Deco mesh, such as:

In some cases, adding more WiFi nodes may be required to achieve that.

With gigabit Internet speed X50 is the right choice and you shouldn't be replacing them with WiFi5 Deco models. That my suggestion is now off the table.

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 they don't have beta firmware for X50 yet, perhaps they have it for a similar Deco model such as X60, and then you will decide if you can swap X50s for X60s and if you want to.