There’s one more option to consider, although it’s not Zwave. But it will update the master status to SmartThings if the auxiliary is used.
As of March 2017, there is now an official cloud to cloud smartthings/Lutron Caseta integration. It does require a Lutron smartBridge. (One bridge can handle several dozen Lutron devices.) Lutron uses its own proprietary protocol, Clear Connect.
Lutron recommends using their wireless pico remote in a wallmount frame as an auxiliary, but you can also use conventional switches and physical traveler wires with some of their models.
The pico itself will be invisible to SmartThings so you can’t use its button presses for anything else, but it will be able to work to control the Lutron master switch.
And Lutron will report to SmartThings whenever the master turns on, regardless of what initiated it.
You can typically buy a bundle with the smartbridge, one inwall dimmer, and one pico for just under $99. After that, a switch plus pico set is typically around $60, although you may be able to find it for a little less. These are widely available in white at Home Depot, Lowes, Amazon, etc. if you want black or other colors, you’ll probably have to use a specialty retailer.
https://www.amazon.com/LUTRON-P-BDG-PKG1W-Caseta-Wireless-Homekit-enabled/dp/B00MXCRAX8/
So that’s just another option for a three-way set up that would work with SmartThings, would also work if the Internet was down (the Dimmer/Pico connection runs locally), and would meet the status update requirement even though it isn’t zwave. So just thought I’d mention it. 
For more detail see the following thread: