Can a thunderbolt 3 cable be used on MacBook 12inch for data transfer?
I am answering my own question. Short answer: NO.

I received my Belkin Thunderbolt 3 cable today (https://www.amazon.com/Belkin-USB-IF-Certified-Thunderbolt-Compatible/dp/B0725DW6D3/ref=sr_1_4?ie=UTF8&amp;qid=1511766135&amp;sr=8-4&amp;keywords=belkin%2Bthunderbolt%2B3&amp;th=1 the 1.6 foot one). By directly connecting my new MacBook Pro and MacBook 12inch, the new MacBook Pro still couldn’t recognize that 12inch MacBook. I did some more research just now, and on this page(https://support.apple.com/en-us/HT201462 scroll down to the bottom), it seems that a USB-C 3.1 cable is the only way that does the work. The shop assistant was right.

I am not sure if it could work with other situations, but unfortunately it doesn’t work with target disk mode. Btw, the cable doesn’t have any problem since my 12inch MacBook was charging when connecting to the new MacBook Pro.