Which is the key belief of Buddhism?

In Buddhism, the three marks of existence are three characteristics (Pali: tilakkhaṇa; Sanskrit: trilakṣaṇa) of all existence and beings, namely impermanence (anicca), unsatisfactoriness or suffering (dukkha), and non-self (anattā). These three characteristics are mentioned in verses 277, 278 and 279 of the Dhammapada (Beliefs-four noble truths) (Daily/weekly rituals-meditation) Holy book-Dhammapada