Question

Two coherent sources of light are three

wavelengths out of phase. does light from these
sources experience constructive interference,
destructive interference, or no interference at all?
explain how you know.

Answer 1

The difference phase = 3 λ

In terms of wavelength the difference of phase is zero so the sources would experience "constructive" interphase at that point.

If the light from the sources differed by 1/2, 3/2, 5/2 wavelengths etc, then the sources would be out of phase at that point.