We report a low-cost and compact extrinsic Fabry-Perot interferometer-(EFPI) based optical fiber sensor for measuring three-dimensional (3D) displacements, including interfacial sliding and debonding during a delamination process. The idea is to use three spatially-arranged EFPIs as the triaxial displacement sensing elements. The sensor consists of an optical fiber component and a mirror component. The fiber component includes three optical fibers, and their corresponding mirrors form the mirror component. Two coincident roof-like metallic structures are used to support the two components, making sure that the endfaces of the fibers and corresponding mirrors maintain a parallel relationship during measurements. As a result, three EFPIs are formed by the endfaces of the optical fibers and their corresponding mirrors. The prototype sensor was first calibrated, and then an experiment monitoring the interfacial sliding and debonding between a long square brick of mortar and its steel base plate support during the drying/curing process was conducted to demonstrate the practicability of the sensor. The experimental results show that our sensor can function continuously for a long period of time. The details obtained from the measured data were also discussed. The robust and low-cost three-dimensional sliding and debonding sensor has a high potential in various applications, especially in structural health monitoring.