USB-C will work with TB-3, but TB-3 will not work with USB-C.
50% interchangeable.
So, a USB-C drive can be connected to a TB-3 equipped computer, but a TB-3 drive cannot be connected to a computer that has only USB-C. For the latest Macs, they support just about anything (and at ridiculous speeds), but USB-C computers (including the 12" MacBook) will not accept a TB-3 external drive unless that drive also has a USB-C port.
To be clear the connectors are 100% interchangeable, but the technology behind those connectors is not. There is a chip on the board at both ends, as well as a chip on the cable at both ends. All those chips negotiate the services offered and shake hands when the services match. If there's no match, then nothing works.