Plasma-facing components (PFCs) with tungsten armor material are expected to withstand the extreme incident heat flux conditions in a nuclear fusion tokomak. In situ measurements of thermal-mechanical behavior under high heat flux (HHF) conditions are important and essential. With this aim, a novel three-dimensional (3D) (stereo) digital image correlation (DIC) diagnostics with an optical mirror adapter in a single optical view port is established for in situ measurements of a full-field strain and deformation profile on PFCs during heat flux testing in the High Heat Flux Test Facility (HHFTF) at the Institute for Plasma Research. In the present work, the thermally induced field deformations of a reflector divertor mock-up under the heat flux conditions of 200 hHF cycles at 5 MW/m2 and 1000 hHF cycles at 7 MW/m2 are measured using the developed 3D-DIC diagnostics. Calibration and validation qualification tests are conducted on the experimental platform as well as numerical COMSOL simulation to ensure the performance and the accuracy of the 3D-DIC diagnostics system. Because of periodic thermal stress, the thermomechanical behavior cases of bonded and de-bonded W-Cu tile interface are captured in situ by measured strain curves and compared with pre-ultrasonic and post-ultrasonic test analyses. Full-field strain and displacement profiles on the reflector divertor mock-up during heat flux testing are also obtained. The 3D-DIC diagnostics results demonstrate the feasibility and accuracy for in situ strain and displacement measurements of PFCs during HHF tests in HHFTF.