Nitrogen dioxide (NO2) is a significant air pollutant with a notable environmental impact, necessitating precise monitoring. However, most existing NO2 sensing materials exhibit low sensitivity and require high-temperature operation. In this study, we employed a hydrothermal and co-precipitation method to synthesize a novel ternary nanocomposite, In2O3/SnO2 nanoparticles (NPs)/fluorine-doped carbon quantum dots (F-CDs). By varying the mass ratio of In2O3 to SnO2 NPs, we investigated the gas-sensing performance of the composite towards NO2. The results demonstrate that when the ratio of SnO2 and In2O3 reaches to 1 : 2, the 2-In2O3/SnO2/F-CDs gas sensor exhibited the highest response, excellent selectivity, and remarkable stability under ultraviolet (UV) light irradiation at room temperature (∼25 °C). Furthermore, the response of the 2-In2O3/SnO2/F-CDs gas sensor displayed a strong linear relationship with the concentration of NO2 gas. At a NO2 gas concentration of 3 ppm, the sensitivity was 238, which is significantly higher than that of In2O3/SnO2 NPs and In2O3 sensors. The response time and recovery time were measured at 62 s and 33 s, respectively. Finally, based on experimental characterization results and band structure analysis, we confirmed the promising effects of ternary nanocomposite in enhancing gas sensing.