USB-C and Thunderbolt 3 are different things. USB-C is a connector defined as part of the USB spec, which was developed in tandem with USB 3.1. TB3 is a different spec, basically an external PCIe interface, that uses the USB-C connector (used to use mini DisplayPort). Typically, TB3 ports are also compatible with USB 3.1, but I don't think that's a requirement.