TRM3 is a device and range performance model for modern thermal imagers with their numerous possible design features. The TRM3 computer program contains two different device models: The Conventional Approach model and the TRM3 Approach model. In the Conventional Approach model, the assumptions are similar to FLIR92. The TRM3 Approach model uses different assumptions in case of fixed pattern noise and of undersampled systems. If the imager is undersampled, an MTDP (Minimum Temperature Difference Perceived) is calculated, rather than an MRTD. This MTDP is used to predict the range performance of undersampled imagers. With the new approach, significantly higher ranges are predicted than those expected with FLIR92. The problem in assessing undersampled thermal imagers with the standard 4-bar test pattern is shortly described. The new approaches in the TRM3 Approach model are outlined and discussed. Examples of measured and predicted MTDP data as well as range data are given. Finally, a short introduction to the TRM3 computer program is given.