Commentary
NUMBER 3: The Quran is quick to speak about deviations which the past communities went through – not to mock them, but to let the Muslims know that we are not immune from the same thing, and we must be vigilant to ensure that we do not allow our faith to become corrupt with inaccurate teachings. 
The point which the Quran mentions in this verse is the erroneous belief of the majority of Christianity – then and even now - claiming THREE Gods – God the Father, God the Son (Jesus), and God the Holy Spirit – but somehow this math of 1+1+1 is still equal to ONE! In this verse, and this is repeated many times in the Quran, Allah shows us those who say that Allah is one of the three in the godhead (trinity in unity) are actually in a state of disbelief in God. 
What is this doctrine – simply put, “The Christian doctrine of the Trinity (Latin: Trinitas, lit. ‘triad’, from Latin: trinus “threefold”) holds that God is one God, but three coeternal consubstantial persons or hypostases - the Father, the Son (Jesus Christ), and the Holy Spirit—as “one God in three Divine Persons”. The three Persons are distinct, yet are one “substance, essence or nature (homoousios).” 
At the First Council of Constantinople (381), the Nicene Creed was expanded, known as Niceno-Constantinopolitan Creed, by saying that the Holy Spirit is worshiped and glorified together with the Father and the Son (συμπροσκυνούμενον καὶ συνδοξαζόμενον), suggesting that he was also consubstantial with them: “We believe in one God, the Father Almighty, the Maker of heaven and earth, and of all things visible and invisible. And in one Lord Jesus Christ, the only-begotten son of God, begotten of the Father before all worlds (æons), Light of Light, very God of the very God, begotten, not made, being of one substance with the Father; (...) And in the Holy Ghost, the Lord and Giver of life, who proceedeth from the Father, who with the Father and the Son together is worshiped and glorified, who spake by the prophets (...).”
The Quran, which came over 600 years after Prophet Jesus ascended into the heavenly realm, sought to eradicate this erroneous belief and educate the Christian nation that there is no god which you should worship other than the ONE God and that this notion of “three in one” is invalid. 
In conclusion, Allah clearly tells those who believe in the trinity that if they do not desist in this belief – obviously after being educated and taught that this is not a correct understanding of God, then those who continue in disbelief will face a painful punishment in the world to come.
Point of Reflection
Islam, as the final religion, and the Quran as the final testament to humanity, speaks about the ideologies of other groups that came before it and comments and critiques them and is not afraid to call out the previous dispensations when they have deviated.