🌠When three people are together, two should not speak secretly to the exclusion of the third.
In doing that, the two that are speaking privately are, either knowingly or unknowingly, causing sadness to enter the heart of the third among them.
Ever eager to bring sadness to the heart of a Muslim, Shaitan will whisper to him and instill doubts into his heart, so that he will become suspicious of the other two, feeling that if they are not plotting against him they are at the very least backbiting him or excluding him, not considering him worthy enough to share their secret. To prevent the Shaitan from instilling such doubts in a Muslim, the Prophet(SAW) said,
"Two people must not speak secretly to the exclusion of the third(among them), for that will make him sad"
But if you are in a gathering of four people, the reason for the prohibition is absent, and so two of you may speak in private, if necessary, to the exclusion of the other two that are present. The prophet(SAW) said,
"If you are three then let not two men (among you) speak to the exclusion of the other, at least not until you mix with people(until you are more than three) , so that you do not make him sad".