The three levels are (1) the everyday world, (2) the actual emptiness of that world, (3) and the middle path that recognizes both of these aspects of reality.
This way of understanding reality gradually became the main Chinese Buddhist teaching about the nature of reality (hence human nature too).
In this doctrine of reality, the Threefold Truth, reality may be characterized as “subtle existence,” meaning that it is both real and empty at the same time—a truth that is not obvious, hence subtle.