The high-performance formaldehyde sensor is an ideal choice for industrial gas detection and environmental gas analysis. In this article, the 3D Ti3C2Tx MXene/rGO/SnO2 aerogle was successfully fabricated via a simple one-step solvothermal route. The composite structures and morphology were characterized by an array of testing techniques. According to the gas measurement, the 3D Ti3C2Tx MXene/rGO/SnO2 sensor exhibited 54.97% sensing response in 10 ppm formaldehyde at room temperature (RT). Not only that, the 3D Ti3C2Tx MXene/rGO/SnO2-based sensor indicated an excellent sensing selectivity, short response and recovery times (2.9 and 2.2 s, separately), good repeatability and stability. A general formaldehyde sensing mechanism with a ternary architectural was proposed and supported through density functional theory (DFT) calculations data, which has been sufficiently shown to be a promising candidate for the design of efficient formaldehyde sensor.