Soil-structure interfaces exist widely in many geotechnical applications and play an important role in soil-structure interaction. Few data have been found on the three-dimensional (3-D) interface behaviors because of the limitation of test devices. A new large-scale test apparatus—the 80-ton 3-D Multifunction Apparatus for Soil-structure interface (3DMAS)—has been professionally designed and developed to investigate the cyclic behaviors of soil-structure interfaces subjected to 2-D and 3-D complex loading conditions. New features of the 3DMAS apparatus include: (1) accurate 3-D loading and control system to allow 3-D interface tests; (2) high loading capability and installation of large test specimens; (3) new measurement method to enable the direct measurement of the structure’s displacement relative to the soil; (4) accurate application of three typical boundary conditions normal to the interface; (5) accurate application of various shear paths in tangential direction; (6) computerized and automated control and measurement systems, allowing forces and displacements to be easily and accurately controlled and measured. The effectiveness and reliability of this apparatus has been verified by a series of 3-D displacement-controlled and stress-controlled direct shear tests conducted on the interface between gravelly soils and structures under the three normal conditions of constant stress, constant stiffness, and constant volume. Typical test results indicate that the 3-D mechanical behavior of soil-structure interfaces is significantly different from the 2-D behavior and that it is necessary to systematically investigate the 3-D shearing behavior of the interface.