This is a good read regarding the differences between Thunderbolt 3 and USB-C.
Thunderbolt 3 is backward compatible with USB-C.
All USB-C devices can be plugged into, and will work in, a Thunderbolt 3 port, but it will transfer data at the slower USB-C speed. An easy thing to remember is that Thunderbolt 3 ports are technically backward-compatible with USB-C devices.
Thunderbolt 3, however, is not (necessarily) USB-C compatible. While it’s true that you can physically plug a Thunderbolt 3 device into a USB-C port, it isn’t guaranteed to work. Some Thunderbolt 3 devices, like power adapters, may charge your USB-C-only laptop, but devices that transfer data probably will not. You’ll likely get a message on your laptop screen that the Thunderbolt 3 device is incompatible with the USB-C port.