When three are together, two must not converse privately, leaving the third out. The Prophet ﷺ said: “When you are three, two should not converse privately excluding the third until you mix with other people, for that would grieve him.” (al-Bukhārī & Muslim).
العربية: عن عبد الله بن مسعود رضي الله عنه أنّ النبيّ ﷺ قال: «إذا كنتم ثلاثةً فلا يتناجى اثنان دون الآخر حتى تختلطوا بالناس، من أجل أن يُحزنه». (البخاري ومسلم).
With three present, don't whisper or speak a language only two share — it hurts the one left out.