A mixing-extruding head apparatus for a three-dimensional (3D) printer for building residential houses and other objects, which enables printing of right-angled corners, the apparatus comprising a container for mixing mortar, the container comprising a funnel (1), a two-part helix (6) inside the container for mixing the mortar for building said objects, wherein the two-part helix (6) is installed on a main axis (2) inside the funnel (1) and the helix (6) comprises an inner metal part (6a) and an outer rubber part (6b), and wherein the helix (6) has at least one thread (turn), and an exit nozzle (14). The bottom part of the exit nozzle (14) is on each of its sides equipped with one of four blades (17), which are controlled by a servo-pneumatic system with computer-controlled commands for building the objects, wherein each of the blades is connected to a piston moved by a pneumatic cylinder; wherein all four pneumatic cylinders are connected with four electromagnetic valves, which generate pneumatic signals.