A three-stage degassing and dewatering device includes a first-stage degasser, a second-stage degasser, an oil drainer, a rod electrode, a dewaterer, and a water drainer. The first-stage degasser implements a first-stage axial-flow type collision buffer degassing and dewatering operation, forming a first-stage crude oil after removing some of the gas and water in the gas-containing and water-containing crude oil. The second-stage degasser implements a second-stage elevated efficient degassing operation, forming a second-stage crude oil after removing the remaining gas in the first-stage crude oil. The rod electrode constructs a dynamic electric field with a high frequency and a high voltage, and implements a third-stage high-frequency and high-voltage rapid dewatering operation together with the dewaterer, forming a qualified crude oil after removing the remaining water in the crude oil emulsion.