It is absolutely not necessary for linear optics to use discrete wavelength. You can set a broadband source, but set the monitor to only record those 3 or any number of specified wavelengths.
If it is nonlinear , then you may need to add 3 different sources centered at the desired wavelengths. Please refer to this example to get quasi-monochromatic source: https://optics.ansys.com/hc/en-us/articles/360041722614-4-level-2-electron-material-model
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