Three-dimensional (3D) emission computerized tomography (ECT) is superior to the "slicing" process of the traditional two-dimensional ECT technology and reconstructs the test zone as a whole. As the projections are not limited to the same horizontal plane, 3D ECT can solve the problems of limited detection positions and assembly errors encountered during actual combustion tests, which is necessary for 3D combustion imaging and testing. In this study, a 3D weight matrix calculation algorithm was developed using a mathematical model of camera imaging in a 3D space and the linear interpolation theory to reduce the data amount and improve the calculation accuracy and efficiency of the weight matrix for 3D ECT. The accuracy of the algorithm was verified via numerical simulations using an algebraic reconstruction algorithm for tomography reconstruction. An ECT system with multiple cameras was established, and the proposed algorithm was used to reconstruct the combustion flame. The results have a significant reference value for improving the accuracy and efficiency of tomographic reconstruction.