One of the very core teachings of the Buddha about the nature of reality can be summarised in three points which are known as the Three Characteristics of Existence. When distinguishing whether a teaching is in accordance to the Buddhadharma (the teachings of the Buddha), whether it is true authentic Buddhist teachings, the 3 Dharma Seals serve as a very useful guideline. All true Buddhist teachings must be in accord with the 3 dharma seals. Why? Because the 3 dharma seals are the nature of reality, and if the teaching is not in accord with the nature of reality, it is falsehood, so they cannot be considered Buddhadharma which literally means 'truths of the Awakened One'.
But not only that... the 3 Characteristics of Dharma is not only meant to be understood theoretically. The dharma seals must be directly experienced from moment to moment by all sincere practitioners, we must bear witness the Emptiness truth in action - the moment ceases as it arises, and there is no-self apart from this, and any identification of self, any sense of aversion and clinging, any desire (see 4 noble truths), based on Ignorance, results in Suffering.
"If you can't understand and see all of the teachings taking place right Now, then it is not true knowledge." - ThusnessWhen Buddhism talk about Emptiness, it can also be understood in terms of the Three Characteristic of Existence, because of these Three Characteristics, all things are devoid of inherent, permanent, solid existence. Emptiness is the ultimate nature of all things and is essential for all Buddhists to not only understand it, but intuitively, directly experience this in every moment. This leads to freedom. Fully realising it is Enlightenment.
I shall share some very well written articles on the Three Dharma Seals, written by a modern Arhat (a person who has attained Enlightenment, Nirvana, and freedom from the samsaric cycles of birth and death), known as Dharma Dan.
Although Mahayana Buddhists in China regarded the three Dharma seals as the Dharma seals of Hinayana, asserting that there is only one Dharma seal (Shi2 Xiang4 Yin4) in Mahayana, the Dharma seal of the ultimate reality, of the true aspect of all phenomena, One Dharma Seal/Emptiness is essentially the same thing as the three dharma seals except it's all-in-one. At a higher level Mahayana's "Emptiness" and One Dharma Seal is a more direct way of expressing reality and is enough if one truly understands it, but still it is important to know about the basics of the 3 dharma seals, and by entering through the door of the three seals one can experience One Reality.
The Three Dharma Seals are so essential that I think every Buddhist practitioner, insight meditators, but not only Vipassana - even if you practise Vajrayana, do Zazen, etc (there's something on that later) should take some time to read it.

Enjoy.
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From the book, Mastering the Core Teachings of the Buddha:
http://www.interactivebuddha.com/Mastering%20Adobe%20Version.pdf (pdf version)
http://www.interactivebuddha.com/Mastering%20Word%20Version.doc (document format)
http://www.interactivebuddha.com (The website)
5. THE THREE CHARACTERISTICSThe Three Characteristics are so central to the teachings of the Buddha that it is almost inconceivable how little attention the vast majority of so-called insight meditators pay to them. They are
impermanence, unsatisfactoriness, and
no-self. I cannot possibly stress enough the usefulness of trying again and again to really understand these three qualities of all experience. They are the stuff from which ultimate insight at all levels comes, pure and simple. They are the marks of ultimate reality. Every single time I say, “understand the true nature of things,” what I mean is, “understand the Three Characteristics.” To really understand them is to be enlightened.
Somehow this exceedingly important message just doesn't typically seem to get through to insight meditators, and thus they spend so much time doing anything but looking precisely moment to moment into the Three Characteristics. They may be thinking about something, lost in the stories and tape loops of the mind, trying to work on their stuff, philosophizing, trying to quiet the mind, or who knows what, and this can go on for year after year, retreat after retreat, and of course they wonder why they don't have more insight yet. This is a tragedy of monumental proportions, but you do not have to be part of it! You can be one of those insight meditators that knows what to do, does it, and finally “gets it” in the grandest sense.
The big message here is: drop the stories, find a physical object like the breath or body or pain or pleasure or whatever, and look into the Three Characteristics precisely and consistently! Drop to the level of bare sensations! This is vipassana, insight meditation, or whatever you want to call it. It is the way of the Buddhas. All the “opening to it,” “just being with it,” “letting it go” and all of that are quite important, as we will see later, but insight meditators must, repeat, must, look into the following: