Ammonia, with its significant hydrogen content, offers a practical alternative to pure hydrogen in marine applications and is easier to store due to its higher volumetric energy density. While Ammonia's resistance to auto-ignition makes it suitable for high-compression ratio engines using pre-mixed charge, its low flame speed poses challenges. Innovative combustion strategies, such as dual-fuel and reactivity-controlled compression ignition (RCCI), leverage secondary high-reactivity fuels like diesel to enhance Ammonia combustion. To address the challenges posed by Ammonia's low flame speed, blending with hydrogen or natural gas (NG) in the low reactivity portion of the fuel mixture is an effective approach. For combustion simulation in engines, it is crucial to develop a chemical kinetics mechanism that accommodates all participating fuels: diesel, Ammonia, hydrogen, and NG. This study aims to propose a kinetics mechanism applicable for the combustion of these fuels together. The mechanism is tailored for engine conditions, including high pressures and temperatures, and diverse chemical species concentrations. To render the mechanism suitable for computationally efficient 3-D Computational Fluid Dynamics (CFD) simulations, it is reduced and contains 82 species and 636 reactions, with N-heptane serving as the surrogate for diesel fuel. The mechanism is tuned using optimization methods to match available experimental data on ignition delay time (IDT) for N-heptane. The prediction of IDT and laminar burning velocity values by the mechanism is validated with available experimental data. Additionally, 3-D CFD and quasi-dimensional multi-zone engine simulations are conducted using the new mechanism to verify engine operating parameters against available experimental data.