In this Letter, we demonstrate and experimentally verify the application of three-core photonic crystal fiber (3C-PCF) for the in-line detection of fully polarized states. We prove the response of 3C-PCF to full-polarization states under multi-core polarization interference through experiments. The sensitivity at 1472 nm is 0.0273 nm/rad, and the linear response is better than 98.9% (the optimal operating wavelength can be designed in the range of 1470 to 1570 nm). With the advantages of an all-fiber integrated system, robustness, and wide wavelength coverage, our design holds great promise for facilitating fiber-optic-integrated polarization meters for optical fiber communication and biomedical diagnostic applications.