The invention discloses a grouping three-roller size-reducing mill, solving the technical problems that a three-roller size-reducing mill is easy to generate the inner hexagon during rolling steel tubes. The grouping three-roller size-reducing mill comprises two rolling stands and three rollers which are arranged on the rolling stands and mutually form an angle of 120 degrees; the two rolling stands respectively form a rolling unit in sequence from the first rolling stand to the last rolling stand along the line production direction; and an included angle of gaps among rollers corresponding to the two rolling stands of each rolling unit is 60 degrees. The grouping three-roller size-reducing mill is characterized in that a plurality of rolling groups are arranged from the first rolling unit to the last rolling unit along the line production direction so as to ensure that the numbers of the rolling units in all rolling groups are equal, and included angles among the gaps of the rollers of each rolling stand of each rolling group and the roller gaps of the rollers of each rolling stand of adjacent rolling group are respectively 30 degrees. The invention improves the quality for reducing sizes of thick-wall steel tubes.