EX7700 - Using 3 of them - but struggling to get them as "one network"

So here's the issue. 

You didn't buy a mesh network. You bought a wifi extender that runs as mesh when ran in extender mode. 

Even in extender mode running mesh it can be an issue because its not a full mesh system with something controlling it. 

Mesh extenders can also have issues when running mesh with more than 1 of them because they can connect to each other and end up in a loop. Because nothings controlling them. 

You're also running them as an access point. The 'Onessid' function works when your running them in extender mode. When you end run around it and set them all with their own ssid or even set one ssid for all of them, its on the device to switch. And devices tend to be sticky and not want to switch. 

I always recommend people move to a full mesh system like Orbi if they're needing more than 1 extender. That way there's a device acting as a controller linking them. Its more stable, better performance, and better roaming. 

Extenders have their place but when you're needing more than 1, a full mesh system is the best.