Macbook 12inch to thunderbolt display?

Short answer: Thunderbolt displays - any generation - are not supported by the 12" MacBook. You must use an Apple computer that contains Thunderbolt hardware within it, which the 12" MacBook does not have.

Longer answer: I actually tried this in the spring of 2017 with a variety of adapters with one of the techs at a local Apple Store with just about all the (Apple-supported) adapters they had onhand that would mechanically fit the Thunderbolt 2 display. With every single adapter, we got a message on the 12" MacBook that we connected an unsupported display. Same thing happened with the newer Thunderbolt monitors produced by LG for Thunderbolt 3.

Conclusion: Even though Thunderbolt 3 connections are the same fit as USB-C (or that Thunderbolt 2 connections are the same fit as mDP) Thunderbolt displays will not work with those ports unless they are explicitly specified as Thunderbolt ports on the computer, meaning the computer contains the necessary hardware within it to make it work with Thunderbolt devices, including Thunderbolt displays.

This means that you have 3 options:

1. You can get a third-party USB-C (non-Thunderbolt - this is important) docking station and a third-party HDMI display, both together comparable to the cost of the Apple-endorsed LG Thunderbolt displays. The USB-C (non-Thunderbolt) dock and HDMI display will work with the 12" MacBook, though I haven't had a chance to test its performance.
2. You can opt for a MacBook Air, MacBook Pro, or some other Apple computer with specified Thunderbolt support, which will either work with or can be adapted to work with any of the Thunderbolt displays.
3. You can just use the 12" MacBook like an iPad Pro that runs Mac OS, which really is what Apple's designers likely had in mind when they created the 12" MacBook - a hyper-portable computer that's not really meant to be used with any computer-style connected accessories; we don't expect our iPads to dock to external displays, right?