When you enter a gathering, do not sit between two persons. Rather sit on their left or right side. Rasulullah [Sallallahu álayhi wa sallam] said, “No one is to sit between two people without their permission.”[Sunan Abí Dáwud]
When two people are kind enough to make room for you to sit between them, do not sit cross-legged or crowd them out.
Ibnul ‘Arabi [Rahimahullah] quotes a sage who said,
“Two persons are consider unfair:
(1) A person to whom advice is offered and he takes offence to it, and
(2) A person who is favoured with a seat in a narrow space and he sits cross-legged.” [Adabul Imlá Wal Istumlá]
When you are seated between two people, do not eavesdrop, unless it is not a private or secret matter. Eavesdropping is a bad habit and a sin.
Rasulullah [Sallallahu álayhi wa sallam] said, “Whoever listens to people’s conversation against their wishes, will be punished by molten lead being poured down their ears on the day of Judgement.” [Sahih Bukhari]
Take note that it is inappropriate for you to whisper to someone sitting next to you if you are in a group of three people. By doing so, you are deserting and isolating the third person which will lead him to entertain the worst of thoughts. Such an act does not befit a Muslim. Rasulullah [Sallallahu álayhi wa sallam] has emphatically negated this as the conduct of a Muslim.
Imam Malik and Abu Dáwud [Rahimahumullah] reported that Rasulullah [Sallallahu álayhi wa sallam] said, “Never will any two (Muslims) whisper to each other in the presence of a third person.” The choice of words by Rasulullah [Sallallahu álayhi wa sallam] (“never will” not “never should”) indicates that such a mistake is not only inappropriate but unimaginable and instinctively despicable.
Sayyiduná ‘Abdullah Ibn ‘Umar [Radiallahu anhuma] was asked, “What if there were four?” “Then it does not matter.” he answered, meaning in that case it is not wrong to whisper or to mutter. [Ibid]