﴿ لَّقَدْ كَفَرَ ٱلَّذِينَ قَالُوٓا۟ إِنَّ ٱللَّهَ ثَالِثُ ثَلَٰثَةٍۢ ۘ وَمَا مِنْ إِلَٰهٍ إِلَّآ إِلَٰهٌۭ وَٰحِدٌۭ ۚ وَإِن لَّمْ يَنتَهُوا۟ عَمَّا يَقُولُونَ لَيَمَسَّنَّ ٱلَّذِينَ كَفَرُوا۟ مِنْهُمْ عَذَابٌ أَلِيمٌ ﴾

“Indeed, the truth deny they who say, "Behold, God is the third of a trinity" - seeing that there is no deity whatever save the One God. And unless they desist from this their assertion, grievous suffering is bound to befall such of them as are bent on denying the truth.”

They are indeed disbelievers those who say ‘God is the third of three’ gods that is He is one of them the other two being Jesus and his mother and they who claim this are a Christian sect; when there is no god but the One God. If they do not desist from what they say when they declare a trinity and profess His Oneness those of them who disbelieve that is those who are fixed upon unbelief shall suffer a painful chastisement namely the Fire.