Is it reliable? I can't seem to find name brand RJ-45 to USB adapters anywhere ...makes me a bit skeptical of the whole thing. Is it just a wire conversion or are there inline resistors/caps/ICs?
There are literally a zillion of these things -- just look on Amazon for "usb ethernet adapter" and you'll have your choice of them.
They are actually very reliable. If you look at most new thin laptops, you'll find that many do not have ethernet ports anymore. They use USB Ethernet adapters instead.
Now, some caveats...
- The 3B+ only has USB2. USB 2 is kinda slow. So, while you can find gigabit usb ethernet adapters, you wont get that speed.
- USB on the Rpi3B+ handles everything... ethernet, usb ports, wifi, SDCard, etc. So, while it will still work, be aware that the USB bus could get saturated. The ethernet port on the rpi3b+ is actually a usb ethernet adapter built into the rpi.
- The rpi CPU may not be beefy enough to handle all of this traffic, especially if you think you'll be trying to shove 100mbit across each interface.
- Double-check me on this one, but I think the entire USB bus on the 3b+ is limited to 41MBps or 328mbps. That really isn't much for 3 network adapters to share, along with everything else crammed onto that bus.
From a reliability standpoint, there's no reason to think it wont be reliable just because it's a usb ethernet adapter. If there's a reliability problem, it will most likely be in the 3b+'s USB bus capacity.
What you are trying to do, conceptually, is a great idea for an rpi. But, in all honesty, I'd suggest you use an rpi 4 instead. It's USB bus is 10x faster (USB3 vs USB2) and it's onboard gig ethernet has been separated from the USB bus. Plus, it has a faster CPU and more memory.
Is it just a wire conversion or are there inline resistors/caps/ICs?
It's not anything like a wire conversion – a computer's USB controller is physically incapable of understanding the Ethernet signal.
Instead, these are fully active devices containing a bridge IC which speaks USB on one side and Ethernet on the other. They receive Ethernet data frames and forward the data through a USB endpoint, and the OS has a driver which presents it as a network interface. Fundamentally this works the same way as a PCI-based Ethernet card would.
(In fact, the built-in Ethernet port on older Raspberry Pi versions was internally attached through USB in basically the same way as these external adapters.)
I can't seem to find name brand RJ-45 to USB adapters anywhere ...makes me a bit skeptical of the whole thing.
The adapters are assembled by various brands, but internally they commonly use chips produced by reputable companies such as ASIX (e.g. AX88179) if you're lucky or Realtek (e.g. RTL8153) otherwise. However, their Linux driver support varies.