In Chinese philosophy, the phrase three teachings (Chinese: 三wikt:教; pinyin: San Jiao) refers to Confucianism, Taoism, and Buddhism when considered as a harmonious aggregate. Some of the earliest literary references to the "three teachings" idea dates back to the 6th century by prominent Chinese scholars of the time. The term may also refer to a non-religious philosophy built on that aggregation.
The phrase also appears as the three teachings harmonious as one (Chinese: 三教合一). It can also refer to a syncretic sect founded during the Ming Dynasty by Lin Zhaoen. In that sect Sanyi Religion, Confucian, Buddhist and Taoist beliefs were combined based on their usefulness in self-cultivation.
Alternatively, in common understanding, three teachings harmonious as one simply reflects the long history, mutual influence, and (at times) complementary teachings of the three belief systems, with little relationship to Lin Zhaoen's sect.
Confucianism is a complex school of thought, sometimes also referred to as a religion, revolving around the principles of the Chinese philosopher Kong Qiu (westernized: Confucius). It was developed in the Spring and Autumn Period during the Zhou Dynasty. Main concepts of this philosophy include Ru (humaneness), righteousness, propriety/etiquette, loyalty, and filial piety, along with a strict adherence to social roles. This is illustrated through the five main relationships Confucius interpreted to be the core of society: ruler-subject, father-son, husband-wife, elder brother-younger brother, and friend-friend. In these bonds, the latter must pay respect to and serve the former, while the former is bound to care for the latter.
The following quotation is from the Analects, a compilation of Confucius' sayings and teachings, written after his death by his disciples. "The superior man has a dignified ease without pride. The mean man has pride without a dignified ease." — Confucius, The Analects of Confucius
This quotation exemplifies Confucius' idea of the junzi (Chinese: 君子) or gentleman. Originally this expression referred to "the son of a ruler", but Confucius redefined this concept to mean behavior (in terms of ethics and values such as loyalty and righteousness) instead of mere social status.
Taoism, or Daoism, is a philosophy centered on the belief that life is normally happy, but should be lived with balance and virtue. Its origin can be traced back to the late 4th century B.C and the main thinkers representative of this teaching are Laozi and Zhuangzi. Key components of Daoism are Dao (the Way) and immortality, along with a stress on balance found throughout nature. There is less emphasis on extremes and instead focuses on the interdependence between things. For example, the yin/yang symbol does not exemplify good or evil. It shows that there are two sides to everything -"Within the Yang there exists the Yin and vice versa." 
The basis of Taoist philosophy is the idea of "wu wei", often translated as "not doing". But, in practice, it refers to an in-between state of "not doing" and "being, but not acting". This concept also overlaps with an idea in Confucianism as Confucius similarly believed that a perfect sage could rule without taking action. Two other assumptions in the Taoist system are 1) any extreme action can initiate a counteraction of equal extremity and 2) excessive government can become tyrannical and unjust, even government created with good intentions.
The following is a quote from the Dao De Jing, one of the main texts in Daoist teachings. "The truth is not always beautiful, nor beautiful words the truth." ― Lao Tzu, Tao Te Ching (Dao De Jing)
Buddhism is a religion that is based on the teachings of Siddhartha Gautama. The main principles of this belief system are karma, reincarnation, and impermanence. Buddhists believe that life is full of suffering, but that suffering can be overcome by attaining enlightenment. Nirvana (a state of perfect happiness) can be obtained by breaking away from (material) attachments and purifying the mind. However, different doctrines vary on the practices and paths followed in order to do so. Meditation serves as a significant part in practicing Buddhism. This calming and working of the mind helps Buddhists strive to become more peaceful and positive, while developing wisdom through solving everyday problems. The negative mental states that are sought to be overcome are called "delusions", while the positive mental states are called "virtuous minds". Another concept prominent in the Buddhist belief system is the Eight-Fold Path. The Eight-Fold Path is the fourth of the Four Noble Truths, which is said to be the first of all Buddha’s teachings. It stresses areas in life that can be explored and practice, such as right speech and right intention.
Though the term "three teachings" is often focused on how well Confucianism, Buddhism, and Taoism have been able to coexist in harmony throughout Chinese history, evidence has shown that each practice has dominated, or risen to favor, during certain periods of time. Emperors would choose to follow one specific system and the others were discriminated against, or tolerated at most. An example of this would be the Song Dynasty, in which both Buddhism and Taoism became less popular. Neo-Confucianism (which had re-emerged during the previous Tang Dynasty) was followed as the dominant philosophy. A minority also claims that the phrase "three teachings" proposes that these mutually exclusive and fundamentally incomparable teachings are equal. This is a contested point of view as others stress that it is not so. Confucianism focuses on societal rules and moral values, whereas Taoism advocates simplicity and living happily while in tune with nature. On the other hand, Buddhism reiterates the ideas of suffering, impermanence of material items, and reincarnation while stressing the idea of reaching salvation beyond.