I need to use the functions in this link:
But I don't know how do I call the singleton in Godot 3.1 Alpha 3
B4DNetwork's answer is correct about accessing singletons. I wanted to mention, just in case you missed it, that using that module requires compiling an export template from source, in order to make the module available via a singleton.
Compiling export templates
Check This Solution
you might use :