If I had a Hopper 3, I could connect a Joey 3 through just an Ethernet cable directly to it from the Hopper 3 to provide connectivity, rather than through RG6? Or do you mean hook up both the Hopper 3 and Joey 3 to the same internet through Ethernet and have them connect together like that?
Both are valid. Let me give you a few examples:
Lets say you are installing a H3 and J3 as you say, and this person has Cat5e throughout the home like in HipKat's comment (who is also a Tech like you, but afaik does not work for Dish directly). When you connect the Hopper to the internet it will grab an IP address from the Modem/Router, lets assign it X.X.1.101 for example, and its working as expected. Ok, now lets install the Joey. On startup, it will attempt to connect to the internet/network by any means it has, for Wired Joeys that's either ethernet or the MoCA network, and for WJ its Ethernet or a paired WAP. If you plug in your coax, a MoCA connection gets established to the Hopper, and if the Hopper has bridging then the internet Modem/Router assigns it an address as well, lets call it X.X.1.102. Keep in mind that until now there has been no actual Hopper-Joey linking going on, its all computer networking. Ok, so the Joey has an address, now what? Well, it will start looking for Hoppers to pair too on the same
subnet (in basic terms, the network the Joey is currently in) which in this case is X.X.
1.X, and what do you know, there is a Hopper on the same network, address 101. And the rest is how you know, it pairs, downloads software, authorizes, etc etc.
Now, lets change the MoCA for a standard Ethernet cable. Doesn't matter if its on the Hopper or a switch, or even the Modem/Router directly, the Joey will get an address on the X.X.1.X network and look for Hoppers. If one is connected, done.
Second example, same situation as above, but the person you are installing for does not have internet. What happens here is that the Hopper takes a Host role, and starts providing IP addresses to any devices that request them like a Modem/Router would, be it via MoCA for wired Joeys or Ethernet for a WJ via the WAP. This is the reason you can install the Hopper system despite having no internet. So now the Hopper is hosting network X.X.
2.101. You can probably tell what happens next, lets connect via MoCA first. The Joey will connect and ask for an address, and the Hopper will assign it X.X.
2.102, then the Joey will look on network X.X.
2.X, find the Hopper, and done. Now, lets replace the MoCA for Ethernet, and pretty much the same happens. Joey gets an address, connects and done.
Now, a few problems with this, like I mentioned above. Instead of using a medium isolated from your internet network, you will have Joeys communicating over the same cables your computers/smart tv's/etc, and if your network is not up to par then you can have issues. Also, if you try to connect a wired Joey via Ethernet and the Hopper is on Wifi, you may experience latency issues because one part of the path between the two devices is wireless and that can introduce problems. Lastly, Bridging is
really important if you do Hopper to Joey directly, because, as the name implies, it bridges the Ethernet and MoCA networks so that there can be communication between the two. If bridging is off, and the Hopper is on network X.X.2.X while the Joey is on X.X.1.X, then no communication. All these are probably reasons Dish doesn't recommend/allow it, because troubleshooting these kind of issues would be a whole new level of crap-tastic to the average tech.