JETNET 3.0 - A versatile artificial neural network package. An F77 package for feed-forward artificial neural network data processing, JETNET 3.0, is presented. It represents a substantial extension and generalization of an earlier release, JETNET 2.0. The package, which consists of a set of subroutines, is focused on multilayer perceptron architectures. As compared to earlier versions it contains a variety of minimization options, measures for monitoring the learning process, limited precision emulation, etc. Also, the reader is provided with a set of guidelines for when to use the different options.