Short answers, pulled from the story.
The three marks of existence are anicca (impermanence), dukkha (commonly translated as suffering or unsatisfactoriness), and anatta (the absence of a lasting self or soul). They are described in the Pali term tilakkhana and the Sanskrit term trilaksana as characteristics of all existence and beings.
Anatta, or anatman in Sanskrit, means that no permanent essence exists in any thing or being, including living beings. Religious studies scholar Alexander Wynne describes it as a "not-self" teaching rather than a "no-self" teaching, and some traditions hold the doctrine applies strictly to the five aggregates rather than to all phenomena.
The Pali Canon, citing the Samyutta Nikaya, states that what is impermanent is dukkha, and what is dukkha is not permanent. Because conditioned things change and cannot be made to last, they generate unsatisfactoriness. Mahasi Sayadaw describes dukkha as "unmanageable, uncontrollable."
Yes. The Ekottarika-agama and Mahayana sources including the Yogacarabhumi-Sastra and The Questions of the Naga King Sagara describe four characteristics, sometimes called the four seals of the Dharma. The fourth seal adds that nirvana is peaceful, a point also found in the Sarvastivada tradition's Samyukta Agama.
Buddhism regards ignorance of the three marks, termed avidya or moha, as the first link in the process of samsara, the cycle of repeated existence and dukkha. Direct insight into the three marks is said to dissolve that ignorance and bring samsara to an end, described in the third of the Four Noble Truths as dukkha nirodha.
The concept of humans being deluded about the three marks, that delusion causing suffering, and insight ending that suffering, is a central theme in the Buddhist Four Noble Truths. The last of the Four Noble Truths leads to the Noble Eightfold Path. The teaching of the three marks in the Pali Canon is credited to Gautama Buddha.