@Opti12 Thanks for the information.

All of the devices that are in the same room as Deco #3 are wireless, yet none of them connect to it and instead are connected to either Deco #2 or the Main Deco which are located on the floor of my home.

I did see others discussing the "Connection Preference" option but, unfortunately, that option does not appear to be available with the Deco X60 setup.

I don't know exactly when this problem started, but Dec0 #3 was using wireless backhaul and some of the wireless devices were connecting to it.  I then got ambitious and ran a Cat6e cable from Deco #2 down to Deco #3 (only required a 75-foot cable, so way under the limit) to utilize the wired backhaul feature.

I think my next step will be to unplug the ethernet cable from Deco #3 and put it back in wireless backhaul mode and see what happens.

  @JoeP63 

https://www.tp-link.com/us/support/faq/3480/

That does work? Check and see if firmware update available.

You could also turn off deco #2 put #3 in wireless backhaul get devices to connect and verify #3 can connect clients.Then set it back up with ethernet from #2 to #3.

However the devices might reconnect back to #2.

This might be a case where your 3rd deco is not required as the coverage from #1 #2, are enough  For those devices that prefer #2. You could just leave #3 off and ready for when u expand the network. 

You could also try turning off beamforming see if that helps clients connect to #3