Uh, its a bit more complicated than that.
USB-C is a connector & cabling standard that can carry many different protocols including USB 1/2/3.1g1/3.1g2, DisplayPort, HDMI and Thunderbolt 3. However, not all devices with USB-C ports will support all of those, e.g. the 12" MacBook doesn't support Thunderbolt or USB 3.1gen2.
Its probably easier to say that Thunderbolt 3 is a
superset of USB-C - Thunderbolt 3 ports will have the Thunderbolt logo and use the USB-C connector, and also support USB 1/2/3.1g1/3.1g2 & DisplayPort 1.2 since those are baked in to the TB3 controller chip. However (if I've got this right) there will be USB-C cables, short passive Thunderbolt cables (which I'm guessing are just USB-C cables certified for Thunderbolt) and
active Thunderbolt cables (with cable controller chips in the plugs needed for top TB3 speeds & TB3 cables more than a few feet long, and which will only connect to other Thunderbolt devices). Then you get on to the distinction between USB-C DisplayPort Alt mode (which can connect directly to DP devices and potentially drive 5k displays via DisplayPort 1.3) and Thunderbolt 3's DisplayPort support (which only does DP1.2, but can drive 5k displays by cramming two DP1.2 cables' worth of data down a single TB cable... but needs a TB display at the other end) and try to decide what a TB3 port in USB-C DisplayPort Alt-mode can do... Whimper.
Also - its as clear as mud whether any existing USB-C port or TB3 port (using the current controller, or built in to forthcoming Kaby Lake CPUs) is going to be able to use the new direct
USB-C-to-HDMI cables recently announced. I don't see how they can since the "HDMI alt mode for USB-C" has only just been announced... which means that some USB-C ports will need active HDMI adapters and new ones will only need passive cables.
USB-C is an exiting idea but it also has the potential to be a can of worms (I just hope that, in practice, they stick to the 3 types of cable above... I've seen talk of multiple grades of USB-C cable apart from the Thunderbolt variants).
This would be a bad year to take up IT tech support