When there are three (persons), two should not converse secretly between themselves to the exclusion of the (third) one. (Using translation from Muslim 2183a)
أَنَّ رَسُولَ اللهِ ﷺ قَالَ إِذَا كَانَ ثَلَاثَةٌ جَمِيعًا فَلَا يَتَنَاجَ اثْنَانِ دُونَ الثَّالِثِ