\(\frac{{hc}}{{{\lambda _3}}} = \frac{{hc}}{{{\lambda _2}}} + \frac{{hc}}{{{\lambda _1}}}\,\,\, \Rightarrow \,\,\,\frac{1}{{{\lambda _3}}} = \frac{1}{{{\lambda _2}}} + \frac{1}{{{\lambda _1}}}\,\,\, \Rightarrow \,\,\,{{\text{v}}_{\text{3}}} = {v_2} + {v_1}\)