How to find concentration of three species from given molar absorptivities
The equation you need would seem to be $\ce{A=\epsilon [C] L}$ where A is the amount of absorption (optical density), L the cell path length, your 'distance light travels' and [C] concentration. As you have 3 species absorbing at the same wavelength you have $\ce{A=(\epsilon_1[C_1] + \epsilon_2[C_2] + \epsilon_3[C_3])L}$. You know L and the $\epsilon$'s but only one of the three concentrations and therefore you do not have enough information to determine the two remaining ones.