@union I had to connect my Deco M5 on my 3rd floor to an ethernet cable to get a good speed. I already had an ethernet cable on the 3rd floor for gaming + HD media player purposes - Wifi would never have been good enough. 

I assume there is no chance you can do that?

Or perhaps get an ethernet cable to the 2nd floor - that could boost the performance of that one and thereby improve the performance of the 3rd floor?

I am using my 3 unit M5 setup in AP mode and all 3 are wired to the modem router.

@union 

Check if your 3rd floor Deco connected to 1st floor Main Deco instead of 2nd floor Deco. This part is not clear from your post. If you find 3rd floor Deco connected to main Deco, that might explain slower speed.

On top of what @harry66 suggested, can you connect 2nd and 3rd floor Decos with Ethernet cable? That will help to improve 3rd floor Deco performance.

Finally, if nothing works and you need speed faster than 40MBit, you might consider a pair of powerline adapters, to use as backhaul for 3rd floor Deco with either 2nd floor or main Deco. 

Note that success of using powerline adapters very dependent on specifics of electric wiring in a house, so get them with return policy you could use if they don't work for you.

@harry66 

Q. I already had an ethernet cable on the 3rd floor for gaming + HD media player purposes - Wifi would never have been good enough. 

I assume there is no chance you can do that?

A. No i dont have the posibility to connect  with an utp ethernet cable on the 3d floor because the building has only power sockets and simple phone sockets and no special ethernet sockets mounted on the wall.

Q. Or perhaps get an ethernet cable to the 2nd floor - that could boost the performance of that one and thereby improve the performance of the 3rd floor?

A. Thats not a bad idea but if i take that way of procedure maybe its better to connect a long length ethernet cable directly to the 3d floor from modem/router.

@Alexandre.

You are right, i omitted to say that throught the deco android app i made sure to connect the 3d floor deco unit to the deco unit of 2d floor.

My just simple logic says that if the main deco and 2d floor deco have equal performance by wireless connection , in the same way the 3d floor deco will 

have pretty close enought performance.. and if not, it would be absolute logically to has some loss and i would happy if it was like 70 Mbps, but not.

A great and clever idea is to use Powerlines,instead of penatrate walls and set new ethernet cable lines.

I have a pair of those and especially the AV 500 . i will try and make a test to see how many fishes i put to the basket.

But the truth is that i wanted to avoid everything of these and thats why i purchased the 3 unit whole home mesh technology  Decos.

Do you think that if i upgrade and replace my equipment with lets say Deco M5 as main Deco or even 3 new units i will get better results?

@union 

Q. Do you think that if i upgrade and replace my equipment with lets say Deco M5 as main Deco or even 3 new units i will get better results?

A. I do not think so. You have adequate Deco hardware for your home network, and the issue appears to be between 2nd and 3rd floor Decos. Replacing Main Deco will not change anything. Replacing all mesh with different Deco units unlikely to bring substantial improvements. Suppose, going M5 will improve mesh performance by 25%, which is a lot, but that would mean just having 50Mbps instead of 40Mbps from 3rd Deco. The whole effort is not worth it.

You can try workarounds and troubleshooting steps, but if you still considering replacing hardware after that, I would suggest trying mesh from different manufacturer instead. Perhaps, they have different logic for communication between nodes, which will somehow address the issue you have.

Two more troubleshooting steps you can try with your Decos:

1. Swap Deco units you have on 2nd and 3rd floors, switching their places. Test download speeds on both after the swap. This will help to eliminate unlikely hardware issue with 3rd floor Deco node.

2. Try different positioning, perhaps 3rd floor Deco node is in suboptimal location. Place 3rd floor Deco on its floor right above 2nd floor Deco, check that signal between them is good and check if that will improve download speed for device connected to 3rd floor Deco.

If it does, you now know where the problem is and could try different places for 3rd floor Deco, those where it still has good signal to 2nd floor Deco and good signal to mobile devices on 3rd floor.

Still, try powerline adapters. Even if it is not most elegant solution, if it works as Ethernet backhaul for your 3rd floor Deco and fixes download speed, that is all that matters. 

@Alexandre. 

First of all , I appreciate your time and thank you a lot for all your suggestions.

Well i tried the powerline solution and the results are much better now. i got 70 Mbps in an almost stable "line" without up&downs.

I checked also the application of Deco  from my phone and showed me that the deco unit is connected wired from the main deco (Master) which i have rename it.

Although the utp ethernet cable of powerline is directly connected to the lan port 2 of my modem/router and not to the second port of the main unit Deco which is connected wired to the lan port 1 of modem/router (i don't know if there will be changes of performance then?)

I don't know if i can achive the absolute (100 Mbps on every floor) but the 70 Mbps let me enough satisfied although the wired electricity distance and the unknown electricity phase

cause i have 3 electricity phases on the electricity board.(Powerlines suppose to work better when are connected to the same phase).

When i will find time i will also check the steps 1 and 2 which are very well mentioned and believe with those i will understand a lot and find the "end of the thread"