This paper presents the results of an experimental investigation of the transient behavior of illuminated triple-junction solar cells (TJSCs). The experimental data have shown that the transient behavior of TJSCs during the transition between the open-circuit point and short-circuit point (SCP), and the maximum-power point and SCP has a complex shape with an underdamped part and a constant current level part, owing to the current mismatch between the subcells. Parasitic capacitance is mainly concentrated on the current branch of the IV-characteristic of TJSC that is the operating range of the sequential switching Shunt regulator (S3R). A new large-signal model that adequately simulates the complex dynamic behavior of the TJSC is obtained. The proposed double-capacitance model of the TJSC takes into account the current mismatch and different dynamic behavior of the subcells. It leads to the fact that the calculated power losses in S3R are significantly less in comparison with the conventional TJSC model application.