The Reaction Mechanism Generator (RMG) project has been in development for over a decade, with the current Python version having begun development in 2008. RMG v1.0 was described in 2016. Here, we are excited to present RMG v3.0, which brings many new features including Python 3 compatibility, heterogeneous catalysis modeling, and new parameter estimation algorithms. With these and other improvements, the codebase has doubled to over 120,000 lines of Python code. Many developments have been focused on improving nitrogen, sulfur, and aromatic chemistry to better model combustion emissions and refining processes. RMG has recently been used successfully to model ethylamine pyrolysis, di-tert-butyl sulfide pyrolysis, hexylbenzene pyrolysis, effect of substituted phenols on ignition delay, PAH formation in methane oxidation, and catalytic combustion of methane. The structure and concept behind RMG has been described previously, 7 so only a brief overview will be given here. RMG is a tool for automatically constructing detailed chemical mechanisms which is largely comprised of three components: