You can instantiate the class by declaring a variable and calling the class as if it were a function:
x = mystuff()
print x.average(9,18,27)
However, this won’t work with the code you gave us. When you call a class method on a given object (x), it always passes a pointer to the object as the first parameter when it calls the function. So if you run your code right now, you’ll see this error message:
TypeError: average() takes exactly 3 arguments (4 given)
To fix this, you’ll need to modify the definition of the average method to take four parameters. The first parameter is an object reference, and the remaining 3 parameters would be for the 3 numbers.