The paper presents a universal back-projection (BP) algorithm for three-dimensional photoacoustic computed tomography (PACT), applicable to planar, spherical, and cylindrical imaging geometries. The BP formula is derived using Green's theorem and involves a solid-angle weighting factor to account for variations in detection views. The algorithm is implemented by projecting measured signals backward on surfaces, with the solid-angle factor ensuring accurate reconstruction. Numerical simulations demonstrate the algorithm's effectiveness, showing good agreement with the true source distribution even in the presence of noise. The solid-angle weighting factor compensates for limited-angle views, and the algorithm is insensitive to random noise. The BP formula can be extended to other inverse-source diffraction tomographies and is a promising tool for PACT applications.The paper presents a universal back-projection (BP) algorithm for three-dimensional photoacoustic computed tomography (PACT), applicable to planar, spherical, and cylindrical imaging geometries. The BP formula is derived using Green's theorem and involves a solid-angle weighting factor to account for variations in detection views. The algorithm is implemented by projecting measured signals backward on surfaces, with the solid-angle factor ensuring accurate reconstruction. Numerical simulations demonstrate the algorithm's effectiveness, showing good agreement with the true source distribution even in the presence of noise. The solid-angle weighting factor compensates for limited-angle views, and the algorithm is insensitive to random noise. The BP formula can be extended to other inverse-source diffraction tomographies and is a promising tool for PACT applications.