This is actually an automations issue.
If you are using the classic app, then any device that smartthings can recognize can be used to arm/disarm smart home manager on the V3 hub. Including several different handheld key fobs And keypads.
Unfortunately, if you are using the new app (“SmartThings ( Samsung connect)“), which you will have to use to set up the V3 hub, then there are no devices that can be used to arm/disarm smart home manager. 
This is a change in “security feature” philosophy. (You also can no longer unlock locks except by either opening the app or on a time based schedule.)
At the present time they have said that the classic app will be going away eventually, but they have not given a timeline for it yet.
At CES 2019 they introduced two new SmartThings devices, a keypad and an “access button“ so it seems likely that those will eventually be available for the security features, but again no timeline and no promises.
You can see the differences between the classic app and the new app in the following thread: ( The thread title is a clickable link)
If you would like to see the feature added to the new app, send a note to support, smartthings does not officially read this forum, which is intended for peer to peer support.
The exception to all this is the SmartThings/ADT model hub which Has its own key fob device that can be used to arm/disarm. But that only works with that specific model.