Yeah, you need to add both as code save and publish you can also test with the simulator.
Just the DTH will make the switch in phone app work in conjunction with physical switch.
Install the smart app will allow you to control the switch with Alexa/GH/ smart tiles.
To do this you need to create 3 simulated switches. https://youtu.be/ro2CisLBLjI and look here
Once you have made 3 simulated switches with network IDs and named them what you want the Echo/GH to say you then go to the Marketplace, SmartApps, +My Apps and load my Hui switch binder. In my Binder App choose the actual Hui switch (that you linked to my DH) for the switch, then for the 3 virtual switches choose the 3 Simulated Switches you just made in the order you want them in the 3 gang switch. Assign a name to it. Discover devices with Echo/GH and you should see your 3 simulated switches come up. You dont need to use the actual switch for voice control only the simulated switches. You only use the actual switch within the Smartthings App to control the 3 gangs with your phone. Now you can voice activate these 3 simulated switches the same as you can a std single smart light bulb.