Apple - Thunderbolt 3 to Ethernet adapter
The ridiculous answer to your question is: buy the Apple Thunderbolt 2 to Ethernet adapter AND the Apple Thunderbolt 2 to Thunderbolt 3 adapter and daisy chain them.
As far as I can tell, that is currently the only way to get your ethernet to show up as Thunderbolt on the newer MacBook Pros.
The only Thunderbolt 3 Ethernet adapters I'm aware of are either part of TB3 docks or really bulky 10GigE Ethernet adapters. A USB-C GigE adapter will do just fine. I got one by Aukey for about 10€ which works pretty much just like the TB2 Ethernet adapter from Apple (it even looks the same). Same speed (100+ MB/sek realworld speed to my NAS), no latency or overhead issues.
We use the Belkin USB-C on all our thunderbolt 3 Macs and get speeds far in excess of the review people. I would expect they either got a bad adapter (it happens - even the best hardware companies can't ship every product perfect - that's what warranty support and service are for - to fix that).
We've never had a case where we needed to get a more expensive 10 GB adapter in our experience. We also get 500 to 700 MBps from the MacBook which only does USB with the Belkin adapter.
I would get those until you have a specific case you can't fix with networking checks and quality switches rather than spend more on an exotic combination of 10 GB adapters that run $200+ from the good vendors:
- https://www.sonnetstore.com/collections/thunderbolt-10gbe-adapters/products/solo-10g
- https://www.sonnetstore.com/collections/thunderbolt-10gbe-adapters/products/twin-10g-thunderbolt3-edition
Sonnet's Thunderbolt 2 adapters do very well in our high bandwidth use cases - MacPro serving files or accessing a SAN as do their thunderbolt to SFP adapters. They are pro supported and designed gear for always on use. We rarely if ever need to put them in a design studio or for ProSumer use, but if you have the funds, these are fun adapters that really work well.
For 95% of our use cases - the Belkin is the adapter to choose.