{There is no private conversation of three but He is the fourth of them} - There is no secret meeting of three, whispering something and conversing privately about it, but He is the fourth of them, meaning: He is knowledgeable of it.
﴿مَا يَكُونُ مِنْ نَجْوَى ثَلاثَةٍ إِلَّا هُوَ رابعهم﴾ مَا يكون من خلْوَة ثَلَاثَة يسرُّون شَيْئا ويتناجون بِهِ، إِلَّا هُوَ رابعهم، أَي: عالمٌ بِهِ.
