The three-electrode submerged arc welding (SAW) is a specially developed technique for long joints of relatively thick plates. The process examined here is used in shipbuilding on 20mm thick plates. The high heat source power (totally approx. 80kW.) allows completing the joint in one run. This paper demonstrates the 3D FE model for thermal-stress analysis of three-electrode SAW process. The main subject of this investigation is monitoring and interpretation of the development of the out-of-plane deformation.