I believe that the built-in eero speed test app only tests the speed between the first (gateway) eero and your ISP. So, as you have noticed, it tells you nothing about what is going on with the WiFi inside your home.

I read somewhere that too many eeros confuses some devices and the network slows as they try to figure out which eero to stay connected to.  Since 1300 sq ft is not that big (relative to 3 eeros covering 4500 sq ft), maybe unplug all but the first eero and see what happens? Then add back gradually if too many dead spots.

Since fast.com is a Netflix thing, it really doesn't like some VPNs. Also some ISPs throttle video speeds, so maybe compare with other speed tests.

Maybe also try out the free app by Ubiquiti - WiFiMan (available for both iOS or Android, doesn't matter) - if you have two devices with that app open and connected to the same WiFi network at the same time, it will offer to test the speed between those two devices. This could help track down where the problem might be.

Also the Google Play Store and the Windows 10/11 Microsoft Store have free versions of "WiFi Analyzer" apps that will show you if all your available channels are being flooded by the neighbors' traffic.  Apple does not offer such apps for privacy reasons.

You may have a lemon but it’s rare. I would take all the eeros down except the gateway. Make it simple. See what type of speeds you are getting then go from there. If that eero is having bad speed, replace it with another eero and do it again. If that eero works then you may have a lemon.

the best thing to do is contact support and give them involved. They don’t seem to really be active on this thread. They can check logs, software version, etc.