The following treatise, “Al-Usool Al-Thalatha” by Shiekh ibn Abdul Wahhab (may Allah have mercy on him) was written in order to convey to every Muslim that which is obligatory upon him to know from his Aqeedah (creed).
When the son of Adam dies and his soul departs and he is placed in his grave, The two angels who are charged with questioning come to him and ask him what he used to believe in in this world. They will ask: who was his Lord, what was his religion and who was his Prophet. If he answers are which what is required of him, then he will be rewarded accordingly, but if he does not answer them, likewise he will be punished accordingly.
If he was one of the righteous, angels with white faces come to him, but if he was one of the evildoers, angels with dark faces come to him.
This is what has been narrated from The Prophet (peace be upon him) in a lengthy hadith narrated by Narrated by Abu Dawood and Ahmad which can be found here
Thus ‘The Three Fundamental Principles’ in which the treatise is titled and which the author discusses, is based upon the three questions the individual will be asked about in his grave; that is ‘Who is your Lord,’ ‘What is your religion’ and ‘Who is your Prophet?’
As for the first Principle ‘Who is Your Lord,’ the author discusses that we are the creation of Allah and that it is He we worship through various acts of worship. The author then goes on to discusses that such acts of worship must be directed to only Allah, and one who does not do such and associates partners in worship alongside Allah, is a Mushrik (associationist), Kaafir (unbeliever).
The Second Principle, that is ‘Knowledge of the Religion of Islam with proofs,’ discusses the levels of knowledge of Islam and that, Islam, Iman, Ihsan.
The Third Principle; ‘Knowledge of your Prophet (peace be upon him), gives a brief biography of The Prophet (peace be upon him) life, and his purpose of Prophet-hood, that is to call people to Tawheed and warn against shirk.
To know more about the three fundamental principles you can read Imam AbdulWahab work on the topic:
The Three Fundamentals Principles - Imam Mohammad Ibn AbdulWahab
There are also some books that explains those principles in details such as:
Sharh Al-Thalathat Usool (Explaining the three principles) - Zayd Ibn Muhammad Ibn Haadee al-Madkhalee
Nawaaqid Al-Islam (نواقض الإسلام) - Nullifiers of Islam - By the Noble Shaykh Muhammad ibn AbdulWahab
Know that the Nullifiers of Islaam are ten.
First: Shirk (associating partners) in the worship of Allaah.
Allaah says: “Verily, Allaah does not forgive that partners be associated with Him in worship (i.e. Shirk), but He forgives what is less than that to whom He wills.” [Surah An-Nisaa: 116]
And He says: “Verily, the one who mixes partners in worship with Allaah (Shirk), then Paradise has been made forbidden for him and his final abode will be the Hellfire. And the wrongdoers will not have any helpers (in Hell).” [Surah Al-Maa’idah: 72]
What falls into this is performing sacrifices to someone other than Allaah, such as the person who offers a sacrifice to the Jinn or to a grave.
Second: Whoever places intermediaries between himself and Allaah, calling unto them and asking intercession from them, and seeking reliance in them, has committed disbelief according to the unanimous agreement (Ijmaa’).
Third: Whoever does not hold the polytheists to be disbelievers, or has doubts about their disbelief or considers their ways and beliefs to be correct, has committed disbelief.
Fourth: Whoever believes that some guidance other than that of the Prophet’s (Peace be upon him) is more complete than his guidance and that someone else’s judgement is better than his judgement, such as the one who prefers the judgement of the Tawaagheet (pl. of Taaghoot; false deities/religions) over his judgement, then he is a disbeliever.
Fifth: Whoever hates something that the Messenger came with, even though he may act on it, has disbelieved.
Sixth: Whoever mocks or ridicules any part of the Messenger’s Religion or its rewards or punishments has committed an act of disbelief.
The proof for this is Allaah’s statement: “Say: ‘Was it Allaah and His verses and His Messenger that you were mocking? Make no excuses, you have disbelieved after having had faith.” [Surah At-Tawbah: 65-66]
Seventh: Sorcery, which includes magic spells that cause a person to hate something he seeks after (sarf) or love something he doesn’t seek after (‘atf). So whoever performs it or is pleased with it being done, has committed
The proof for this is Allaah’s statement: “And neither of these two (angels) would teach anyone until they had first said to them: ‘We are only a trial (for the people), so do not commit disbelief.’” [Surah Al-Baqarah: 102]
Eighth: Supporting and assisting the polytheists against the Muslims.
The proof for this is Allaah’s statement: “And whoever amongst you takes them (i.e. the disbelievers) as allies and
protectors then he is indeed from among them. Verily, Allaah does not guide a wrong-doing people.” [Surah Al-Maa’idah: 51]
There is also a detailed explanation for the Nullifiers of Islam by Imam Muhammad bin ‘Abdil-Wahhaab here