Three-phase solid bowl screw centrifuge having a. a rotatable drum and a screw arranged in the drum, b. at one axial end of the drum, at least one solid material discharge and, at its other axial end, at least two or more liquid outlets for liquid phases of different densities - a lighter liquid phase and a heavier liquid phase, c. one liquid outlet having a skimmer disk arranged in a skimmer chamber, and the other liquid outlet being constructed in the fashion of an overflow, characterized in that d. two regulating disks (11, 12), preferably of an identical inside diameter, being disposed in front of the skimmer disk, which extend radially from the outside toward the inside and between which a siphon disk dips which extends in the skimmer chamber from its interior circumference toward the exterior, e. so that, between the siphon disk and the skimmer disk as axial boundaries, the inside radius of the lighter liquid phase in this axial area and the inside shell in the skimmer chamber, an annual chamber is formed during the operation, f. into which annular chamber at least one fluid pipe leads for changing the pressure in the annular chamber, by way of which fluid pipe the pressure in the annular chamber can be changed, in order to change the separating zone and/or the pool depth in the drum, g. wherein first and second axial openings (21, 22; 25, 26) are provided in the drum lid and wherein a separating-plate-type separating weir being assigned to the first or the second openings, h. and wherein a design of the separating weir is provided in such a way that the heavier liquid phase can be guided by way of the separating-plate-type separating weir into at least one discharge space in which at least one discharge pipe penetrating the drum shell is inserted as the overflow. a. a rotatable drum and a screw arranged in the drum, b. at one axial end of the drum, at least one solid material discharge and, at its other axial end, at least two or more liquid outlets for liquid phases of different densities - a lighter liquid phase and a heavier liquid phase, c. one liquid outlet having a skimmer disk arranged in a skimmer chamber, and the other liquid outlet being constructed in the fashion of an overflow, d. two regulating disks (11, 12), preferably of an identical inside diameter, being disposed in front of the skimmer disk, which extend radially from the outside toward the inside and between which a siphon disk dips which extends in the skimmer chamber from its interior circumference toward the exterior, e. so that, between the siphon disk and the skimmer disk as axial boundaries, the inside radius of the lighter liquid phase in this axial area and the inside shell in the skimmer chamber, an annual chamber is formed during the operation, f. into which annular chamber at least one fluid pipe leads for changing the pressure in the annular chamber, by way of which fluid pipe the pressure in the annular chamber can be changed, in order to change the separating zone and/or the pool depth in the drum, g. wherein first and second axial openings (21, 22; 25, 26) are provided in the drum lid and wherein a separating-plate-type separating weir being assigned to the first or the second openings, h. and wherein a design of the separating weir is provided in such a way that the heavier liquid phase can be guided by way of the separating-plate-type separating weir into at least one discharge space in which at least one discharge pipe penetrating the drum shell is inserted as the overflow.