Rabbi Shimon said: “... three who ate at one table and did speak words of Torah there, it is as if they had eaten from the table of G-d...” (Ethics 3:3)
Three together can recite “let us bless,” the opening phrase of the zimun, and in this way they form a “pool” of blessing so that each person partaking of the meal can draw off water according to his needs. But this must be preceded by words of Torah which enable them to form this pool of blessing. (Tzemach Tzedek’s Or HaTorah)