The three marks of existence, anicca, dukkha, anatta, are the characteristics Buddhist teaching ascribes to conditioned phenomena. What the tradition treats as liberating is their direct perception rather than assent to them as propositions.
Anicca, impermanence: everything conditioned arises and passes; there are processes, not persisting things.
Dukkha, usually rendered "suffering" but better rendered unsatisfactoriness or unease: conditioned things cannot yield lasting satisfaction, precisely because they change. The term covers ordinary pain, the suffering produced by change, and the deeper unsatisfactoriness of conditioned existence as such. It is a claim about the structure of experience, not the assertion that everything is painful; the tradition is explicit that there is pleasure, and its point is that pleasure does not last and cannot be made to.
Anatta, non-self: no permanent, unchanging, independent self is to be found in or behind the constituents of a person. This is the most distinctive of the three and marks Buddhism's break with the Upanishadic atman.
The scope of the three differs, and the difference is technical but important. Impermanence and unsatisfactoriness are predicated of conditioned things. Non-self is predicated of all dhammas whatever, including nirvana, which is unconditioned and therefore neither impermanent nor unsatisfactory but is still not a self. The verse formula in the Dhammapada marks the difference by changing exactly one word in the third line, and the commentaries make much of it.
Anatta is not the claim that there is no person. Buddhist tradition consistently affirms conventional personal identity, moral responsibility and continuity across lives, and denies only a permanent unchanging essence underlying them. The Milindapanha's analysis of the chariot is the standard illustration: the chariot is not the axle, the wheels or the pole, nor is it found apart from them, nor is it simply the collection, and yet "chariot" is a perfectly serviceable word. What exactly is being denied has been argued about within Buddhism from the beginning. The Pudgalavada school held a "person" neither identical with nor different from the aggregates and was criticised by every other school for it; and modern scholars continue to divide over whether the earliest texts deny a self or decline to answer the metaphysical question, the Ananda Sutta, in which the Buddha declines to answer whether there is a self, being the passage on which that argument turns.