Hakeemul Ummah Hazrat Moulana Ashraf Ali Thaanwi Sahib رحمه الله has said that it is mentioned in the Hadīth that in a group of three people, two should not converse secretively. This is a wonderful teaching for society. If in a group of three, two persons converse secretively, the third one may feel that they are gossiping about him or they consider him to be unworthy. However, if there are four in a group, then this distress will be obviated.
(SOURCE: MALFOOZAAT OF HAZRAT MOULANA ASHRAF ALI THAANWI SAHIB رحمه الله, PART 6)