The magnitude of CO₂ emissions from estuaries approximates the CO₂ absorption by the continental shelf seas, indicating the important role of estuaries in the global carbon cycle. The global estuarine CO₂ source was estimated from 0.1 to 0.6 PgC yr^-1, with large uncertainties mostly due to the heterogeneity and paucity of observation data around the world. Here, we investigated three tropical estuaries with different hydrodynamic settings in the northern and northeastern Hainan Island during the summer of 2022. Underway equilibrator-NDIR system was used to measure the pCO₂ gradient between the atmosphere and surface water along with other physic-chemical parameters. The pCO₂ ranges of 423~825 uatm, 342~564 uatm, and 70~633 uatm were observed in Dongzhai Harbor, Qinglan Harbor, and Xiaohai, respectively. High spatial and temporal variations of pCO₂ were revealed with a general decreasing trend from the river mouth to the estuarine zone to the adjacent coastal sea. Tidal mixing was the dominant control over the effects of biological activities and riverine input in Dongzhai Harbor and Qianglan Harbor. However, the primary production, riverine input, and tidal mixing all significantly contributed to the pCO₂ dynamics in Xiaohai. The tidal stage is a useful indicator for the interpretation of observation data in tropical estuaries. The CO₂ fluxes were estimated in multiple divisions of each estuary. Our study provides solid observation data and new insights on air-sea CO₂ exchange and its major controls in three tidal estuaries in the tropic region, which may help to further constrain the magnitude of global estuarine CO₂ emissions.