Один - тайна, два - полтайны, три - нет тайны

See Всему свету по секрету (В) Cf.:

A secret shared is no secret (Br.). Secret shared is no secret (Am.). Three may keep counsel if two be away (Am.). Two can keep a secret if one is dead (Am.). Two may keep counsel if one be away (if one of them's dead) (Br.). When three know it, all know it (Br.)