1. A three-dimensional measuring method comprising the steps of: obtaining design data of a measurement object; obtaining a first image of the measurement object taken by a first camera and a second image of the measurement object taken by a second camera from a viewpoint different from the first image; detecting a plurality of line sections of the measurement object from the first and second images and selecting at least a first line section and a second line section from the plurality of line sections; using a stereo method to calculate a formula of a three-dimensional line of the first line section; using the formula of the three-dimensional line of the first line section and a geometric constraint condition for the first line section obtained from the design data of the measurement object to calculate a formula of a three-dimensional line of the second line section; and using the formulas of the three-dimensional lines of the first and second line sections to measure a three-dimensional position or pose of the measurement object.