Aims. Although the real shapes and trajectories of erupting solar prominences in three dimensions have been intensively studied, the three-dimensional (3D) shapes of stable prominences before eruptions have not been measured accurately so far. We intend to make such a measurement to constrain 3D prominence models and to extend our knowledge of prominences. Methods. Using multiperspective observations from the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory (SDO) and the Extreme Ultraviolet Imager on board the Solar Terrestrial Relations Observatory (STEREO), we reconstructed 3D coordinates of three stable prominences: a quiescent, an intermediate, and a mixed type. Based on the 3D coordinates, we measured the height, length, and inclination angle of the legs of these prominences. To study the spatial relationship between the footpoints of prominences and photospheric magnetic structures, we also used the Global Oscillation Network Group Hα images and magnetograms from the Helioseismic and Magnetic Imager on board the SDO. Results. In three stable prominences, we find that the axes of the prominence legs are inclined by 68 ± 6° on average to the solar surface. Legs at different locations along a prominence axis have different heights with a two- to threefold difference. Our investigation suggests that over 96% of prominence footpoints in a sample of 70 footpoints are located at supergranular boundaries. The widths of two legs have similar values measured in two orthogonal lines of sight. We also find that a prominence leg above the solar limb showed horizontal oscillations with larger amplitudes at higher locations. Conclusions. With a limited image resolution and number of cases, our measurement suggests that the legs of prominences may have various orientations and do not always stand vertically on the surface of the sun. Moreover, the locations of prominence legs are closely related to supergranules.