Can I use a Thunderbolt 3 cable in place of a USB-C cable? I need to connect my MacBook to a USB-C dock but the included USB-C cable isn't long enough. I have a several longer Thunderbolt 3 cables, so I wanted to know if I could connect the USB-C dock via a Thunderbolt 3 cable? 

I realize I cannot do it the other way around, but I have never seen anywhere where it said you cannot do it the way I am proposing.

I'm using the CalDigit USB-C Dock.
Since Thunderbolt 3 is a "multi-plexing" of several signals including USB 3.1, you can use a Thunderbolt 3 cable in place of a USB-C cable.   As you stated, it can't be done the other way around.