The design of multi-junction solar cells is guided by both the theoretical optimum bandgap combinations as well as the realistic limitations to materials with these bandgaps. Nowadays, triple-junction III-V multi-junction solar cells are commonly used as GaAs, InGaAs; InGaP ... In this work, we are interested in studying triple junctions based on thin-film solar cells Cu(In1-xGax) Se2, CuInSe2, and CuGaSe2 quaternaries using Silvaco ATLAS software. Incorporating Cu(In0.34Ga0.66) Se2 as an absorber in the middle sub-cell increased the open-circuit voltage by 0.72 V. The highest cell efficiency is 20.89 % (Voc = 2.33 V, Jsc = 9.97 mA/cm2, FF = 89.62%). This triple-junction solar cell demonstrates the potential and limitations of future improvements when voltage and current are considered.