Of course, in Python, you can slice with [start:end:step], so it is valid to do list[:3:3] for double the happiness. Not that you'd ever practically do so.
You're right, it would return a list containing the first element of the sliced list.
So:
lst[:3:3] == [lst[0]]
Well, technically a sequence containing the first element of whatever sequence you sliced. Take advantage of implementing magic methods and you too can abuse slice notation with your classes.