In 1984, what are the three sacred principles of Ingsoc?

The three sacred principles of Ingsoc are Newspeak, doublethink, the mutability of the past. Newspeak is the official language of Oceania and the totalitarian regime's attempt to alter human thought and completely eradicate political dissent. Each year, the Newspeak dictionary becomes smaller and smaller as government agents delete adjectives and incendiary words while they combine opposite words that can be used to describe contradicting thoughts.
The illogical Newspeak words directly connect to the second principle of doublethink, which is the ability to believe two contradicting ideas simultaneously and accept whatever information the Party declares. The Party attempts to control reality via the practice of doublethink. Essentially, the Party can make false, contradictory claims and the public with fully accept, believe, and embrace the information by practicing doublethink.
The third principle is the mutability of the past, which is the government's ability to continually alter historical documents that favor their current policies. One of the Party's slogans reads,

Who controls the past . . . controls the future: who controls the present controls the past. (Orwell, 44)

Winston Smith works in the Ministry of Truth, where he alters and fabricates historical documents to coincide with the Party's current policies. These three principles allow the Party to control, manipulate, and oppress the population of Oceania.

The three main principles of Ingsoc (English Socialism) are expressed in the slogans WAR IS PEACE, FREEDOM IS SLAVERY, and IGNORANCE IS STRENGTH. The significance of these principles, and the reasons for the slogans expressing them, become increasingly apparent to Winston as the story of 1984 progresses.
The Party's reasoning (if one can call it that) is that the population can be controlled and manipulated by destroying its power to think and understand reality. In dictatorial regimes, such as the Communist and Fascist countries of Orwell's own time, the rulers always attempted to obliterate the concept of objective truth. Truth, to them, becomes not what people objectively observe as occurring, but instead whatever the government or the "Party" says it is. Orwell's novel is a projection into the future of what was already happening in the nightmare world of the 1930's and 40's. The statements that "War is Peace," and "Freedom is Slavery," since they are direct self-contradictions and make no sense, are intended to negate the concept of logic. They are also examples of doublethink, the ability to hold two contradictory ideas in the mind and believe both of them simultaneously, as the Party desires people to do, including its own members.
In 1984, the scene in which, in the middle of a rally, the speaker suddenly declares that Oceania is at war with Eastasia and has always been at war with it and is in alliance with Eurasia and has always been—the direct opposite of the truth—demonstrates the Party's manipulation of facts ad absurdum. To the Party, truth is whatever it declares truth to be, and people are expected to believe it in spite of what their own senses and memories tell them, just as they are expected to believe that opposites such as freedom and slavery are the same thing. This is the 1984-version of "alternative facts" being presented as reality.
The third slogan, "Ignorance is Strength," is not as much of a direct self-contradiction as the first two, but it nevertheless demonstrates the same principle—that it is desirable for people not to have knowledge. If people are kept in a state of ignorance (which the Party considers strength), then they can be manipulated and controlled to whatever degree the Party wishes. As O'Brien reveals during Winston's interrogation and torture, the Party's sole motivation is power. In other words, the old ideas about creating an egalitarian society and freedom for the working class are a sham, and this is obvious when one considers the principles embedded in Ingsoc.