Genkir3D is a software package that facilitates the implementation of new 3-D Kirchhoff algorithms. It enables the user to specify a new Kirchhoff operator by only writing a function that computes the kinematics and the amplitudes of the summation surfaces, in addition to the maximum data frequency for anti-aliasing. All the practical issues related to data interpolation, data filtering, geometry handling, and limited memory usage, are efficiently and flexibly taken care by the package. The input and the output are data sets in Sep3D data format, and therefore can have arbitrary trace geometry. The package is developed in Fortran 90.