Thunderbolt 3 compatible with Apple Thunderbolt Display
Looking at the Thunderbolt 3 FAQ, it states the following which should apply to all Thunderbolt-labeled Display Port displays.


  Thunderbolt 3 ports are fully compatible with DisplayPort devices and cables.


and


  Products built to Thunderbolt and Thunderbolt 2 specifications will work with Thunderbolt 3 via an adapter.