إِذَا كُنْتُمْ ثَلَاثَةً، فَلَا يَتَنَاجَى اثْنَانِ دُونَ الْآخَرِ حَتَّى تَخْتَلِطُوا بِالنَّاسِ؛ مِنْ أَجْلِ أَنْ يُحْزِنَهُ
Translation: When there are three of you, two should not whisper privately to the exclusion of the third until you are among other people, lest it cause the third person distress.
Source: البخاري ومسلم
ثَلَاثَةً means “three people,” يَتَنَاجَى refers to private whispering, and دُونَ الْآخَرِ means “excluding the other person.” حَتَّى تَخْتَلِطُوا بِالنَّاسِ shows that the concern is a small gathering in which one participant is isolated.
Private conversation may sometimes be needed. But when only three are present, two whispering and excluding the third can produce suspicion or hurt. The report itself states the reason: مِنْ أَجْلِ أَنْ يُحْزِنَهُ, “lest it make that person sad.”