In Buddhism, the three marks of existence are three characteristics (Pali: tilakkhaṇa; Sanskrit: त्रिलक्षण, trilakṣaṇa) of all existence and beings, namely impermanence (aniccā), unsatisfactoriness or suffering (duḥkha), and non-self (anattā). These three characteristics are mentioned in verses 277, 278 and 279 of the Dhammapada. That humans are subject to delusion about the three marks, that this delusion results in suffering, and that removal of that delusion results in the end of suffering, is a central theme in the Buddhist Four Noble Truths and Noble Eightfold Path.
According to Thich Nhat Hanh, the three seals are impermanence, non-self and nirvana. He says in "The heart of the Buddha's Teaching" that "In several sutras the Buddha taught that nirvana, the joy of completely extinguishing our ideas and concepts, rather than suffering, is one of the Three Dharma Seals."
In the sutra "The Questions of the Nāga King Sāgara" Sāgaranāgarājaparipṛcchā these four marks are defined as:
all compounded phenomena are impermanent (anitya);
all contaminated phenomena are suffering (duḥkha);
all phenomena are without self (anātman);
nirvāṇa is peaceful/peace (śānta/śānti).
In the Samyukta Agama a different formulation is made, in which the Buddha taught impermanence, nonself, and nirvana as the Three Dharma Seals. Here nirvana replaces dukkha as the Third Dharma Seal  :
nirvana - "The joy of completely extinguishing our ideas and concepts, rather than suffering, is one of the Three Dharma Seals."
Impermanence (Pali anicca, Sanskritanitya) means that all conditioned things (saṅkhāra) are in a constant state of flux. Buddhism states that all physical and mental events come into being and dissolve. Human life embodies this flux in the aging process and the cycle of repeated birth and death (Samsara); nothing lasts, and everything decays. This is applicable to all beings and their environs, including beings who are reborn in deva (god) and naraka (hell) realms. This is in contrast to nirvana, the reality that is nicca, or knows no change, decay or death.
Dukkha (Sanskrit duhkha) means "unsatisfactoriness, suffering, pain". The dukkha includes the physical and mental sufferings that follows each rebirth, aging, illness, dying; dissatisfaction from getting what a being wishes to avoid or not getting the desired, and no satisfaction from Sankhara dukkha, in which everything is conditioned and conditioning, or because all things are not experienced as impermanent and without any essence.
Anatta (Sanskrit anatman) refers to the doctrine of "non-self", that there is no unchanging, permanent Self or soul in living beings and no abiding essence in anything or phenomena.
While anicca and dukkha apply to "all conditioned phenomena" (saṅkhārā), anattā has a wider scope because it applies to all dhammā without "conditioned, unconditioned" qualification. Thus, nirvana too is a state of "without Self" or anatta. The phrase "sabbe dhamma anatta" includes within its scope each skandha (aggregate, heap) that compose any being, and the belief "I am" is a mark of conceit which must be destroyed to end all Dukkha. The Anattā doctrine of Buddhism denies that there is anything called a 'Self' in any person or anything else, and that a belief in 'Self' is a source of Dukkha. Some Buddhist traditions and scholars, however, interpret the anatta doctrine to be strictly in regard to the five aggregates rather than a universal truth. Religious studies scholar Alexander Wynne calls anattā a "not-self" teaching rather than a "no-self" teaching.
In Buddhism, ignorance of (avidyā, or moha; i.e. a failure to grasp directly) the three marks of existence is regarded as the first link in the overall process of saṃsāra whereby a being is subject to repeated existences in an endless cycle of suffering. As a consequence, dissolving that ignorance through direct insight into the three marks is said to bring an end to saṃsāra and, as a result, to that suffering (dukkha nirodha or nirodha sacca, as described in the third of the Four Noble Truths).
Gautama Buddha taught that all beings conditioned by causes (saṅkhāra) are impermanent (anicca) and suffering (dukkha), and that not-self (anattā) characterises all dhammas, meaning there is no "I", "me", or "mine" in either the conditioned or the unconditioned (i.e. nibbāna). The teaching of three marks of existence in the Pali Canon is credited to the Buddha.
The Greek philosopher Pyrrho traveled to India with Alexander the Great's army, spending approximately 18 months there learning Indian philosophy from the Indian gymnosophists. Upon returning to Greece Pyrrho founded one of the major schools of Hellenistic philosophy, Pyrrhonism, which he based on what appears to have been his interpretation of the Three marks of existence. Pyrrho summarized his philosophy as follows:
"Whoever wants to live well (eudaimonia) must consider these three questions: First, how are pragmata (ethical matters, affairs, topics) by nature? Secondly, what attitude should we adopt towards them? Thirdly, what will be the outcome for those who have this attitude?" Pyrrho's answer is that "As for pragmata they are all adiaphora (undifferentiated by a logical differentia), astathmēta (unstable, unbalanced, not measurable), and anepikrita (unjudged, unfixed, undecidable). Therefore, neither our sense-perceptions nor our doxai (views, theories, beliefs) tell us the truth or lie; so we certainly should not rely on them. Rather, we should be adoxastoi (without views), aklineis (uninclined toward this side or that), and akradantoi (unwavering in our refusal to choose), saying about every single one that it no more is than it is not or it both is and is not or it neither is nor is not.
^Richard Gombrich (2006). Theravada Buddhism. Routledge. p. 47. ISBN978-1-134-90352-8., Quote: "All phenomenal existence [in Buddhism] is said to have three interlocking characteristics: impermanence, suffering and lack of soul or essence."
^Ulrich Timme Kragh (editor), The Foundation for Yoga Practitioners: The Buddhist Yogācārabhūmi Treatise and Its Adaptation in India, East Asia, and Tibet, Volume 1 Harvard University, Department of South Asian studies, 2013, p. 144.
^[a] Christmas Humphreys (2012). Exploring Buddhism. Routledge. pp. 42–43. ISBN978-1-136-22877-3. [b] Brian Morris (2006). Religion and Anthropology: A Critical Introduction. Cambridge University Press. p. 51. ISBN978-0-521-85241-8., Quote: "(...) anatta is the doctrine of non-self, and is an extreme empiricist doctrine that holds that the notion of an unchanging permanent self is a fiction and has no reality. According to Buddhist doctrine, the individual person consists of five skandhas or heaps - the body, feelings, perceptions, impulses and consciousness. The belief in a self or soul, over these five skandhas, is illusory and the cause of suffering." [c] Richard Gombrich (2006). Theravada Buddhism. Routledge. p. 47. ISBN978-1-134-90352-8., Quote: "(...) Buddha's teaching that beings have no soul, no abiding essence. This 'no-soul doctrine' (anatta-vada) he expounded in his second sermon."