@DrGeo So I found a somewhat inelegant solution. I deleted the third unit from the network.  I connected the second unit to a 50 foot extension cord to the second unit and moved it as close as possible to the third unit. Then I added the third unit to the network again and it connected to the second unit. Solid signal and bandwidth for all three now. 

i shouldn't have to do this. Why can't TP-link fix the software? The support is awful. There is no way to send a message to them. The Deco is working now, but there is no way I will ever recommend this compny

  @DrGeo 

There are no permanent solutions for that issue for Deco X20 yet, but in your case there are two options:

1. Deco X50 and X55 allow to configure how Satellite Deco should connect. See How to select a preferred signal source on the Deco App

If X20s are still under return policy, you could return them and give TP-Link another chance, with X50/X55. They are similar hardware and shape to X20, with a bit better specs.

2. To make your solution more reliable, so that it survives Deco reboots and WiFi backhaul signal fluctuations, you could set second floor Deco as Main Deco. This will force studio Deco connect to it every time.

For that to work, you must have ISP router in your house and have to run Deco in Access Point mode: How to set up Deco to work in Access Point mode

This is how I configured my Deco mesh to deal with that same problem. If you can't run Deco in AP mode, this option won't work for you.

This just keeps getting worse. I've upgraded my Internet from 50 to 150 Mbps. Testing the connection using the app shows that's what is coming to the main Deco router. But all devices are only getting 45 up and down. I rebooted the system and for one test my iPhone got 150 up and down, but just a couple of minutes later it was back to 45. Now the same thing happened to the 3rd floor unit. It lost the WIFI connection so I connected to the second floor unit with an Ethernet cable. First test showed 150 up and down,  but all subsequent tests are back to 45. 

I'm really starting to think this system is just a POS.