As you’ve noticed, they all do the same thing.
asyncio.async had to be replaced with asyncio.ensure_future because in Python >= 3.5, async has been made a keyword[1].
create_task‘s raison d’etre[2]:
Third-party event loops can use their own subclass of Task for interoperability. In this case, the result type is a subclass of Task.
And this also means you should not create a Task directly, because different event loops might have different ways of creating a “Task”.
Edit
Another important difference is that in addition to accepting coroutines, ensure_future also accepts any awaitable object; create_task on the other hand just accepts coroutines.