Simplified resist models are desired for fast simulation of resist profiles over large mask areas. The Lumped Parameter Model was originally developed as one such model. However, the LPM model has been limited to 2D resist simulations of 1D aerial image slices with positive tone resists. In this paper we present a modified Lumped Parameter Model applicable to 3D resist simulations of both positive and negative tone resists. In addition several new LPM parameters are introduced that further improve accuracy. The derivation of the 3D LPM model, rationale for including the new parameters, and simulation results using the new model are given.