The term 'Fill the Tank, with 'UP'' is a phrase used to describe the process of filling a vehicle's GAS tank with fuel. The crossword solution for this term is GAS, which is a three-letter word. GAS is a type of fuel that is used in many vehicles, and it is typically purchased from a GAS station. Filling the tank with GAS is an important part of vehicle maintenance, as it ensures that the vehicle has enough fuel to run properly.