Can you use a Thunderbolt 3 cable to connect two Macbook 12-inch in target disk mode?
The Macbooks connector is only USB-C (USB 3.1 Gen 1), which is not Thunderbolt 3. 


You can't connect in target disk mode via a thunderbolt 3 cable.
You can use USB-C data cable to connect in USB target disk mode. 


(Not that this helps, but the Macbook Pro has native thunderbolt 3 support, and would work in this manner.)

You also can use migration assistant over ethernet if you have a usb-c ethernet on both ends or WiFi, if moving accounts is your ultimate goal.