The Threefold Truth The Tiantai school took up the principle of The
Threefold Truth, derived from Nāgārjuna:
Phenomena are empty of self-nature, Phenomena exist provisionally from
a worldly perspective, Phenomena are both empty of existence and exist
provisionally at once. The transient world of phenomena is thus
seen as one with the unchanging, undifferentiated substratum of
existence. This doctrine of interpenetration is reflected in the
Tiantai teaching of three thousand realms in a single moment of
The Threefold Truth has its basis in Nāgārjuna:
All things arise through causes and conditions. That I declare as
emptiness. It is also a provisional designation. It is also the
meaning of the Middle Path
I am far from qualified to answer this question. But conceptually I believe it's not too difficult to understand the Three Truths, essentially it boils down to the following:
First, understanding that things are empty. Based on dependent origination, we realize that all phenomena that arises are subject to cessation. This is because that is, this is not because that is not. Hence, based on Nagarjuna designation, it is seen as empty.
However, seeing everything as empty doesn't tell us what really goes on. So we at least need to know how to make use of this emptiness. We need to be able to turn our ignorance and delusion into wisdom. We need to perceive the effects of the Four Noble Truths, see the Dharma from Suffering to End of Suffering. We need to become enlightened. We also need to avoid the dangerous pitfall of thinking there is nothing at all and become annihilistic.
Finally when we have attained some wisdom, we also need to realized that whatever attainments we have made, is too impermanent and subject to change. This realization is the middle way. That things are neither merely empty nor truly existent. This is the Middle Way between annihilism and eternalism.