Three identical monochromatic points sources of light emit light of wavelength $\lambda$ coherently and in phase with each other. They are placed on the x-axis at the points x = – d, 0 and d. find the minimum value of d/$\lambda$ for which there is destructive interference with almost zero resultant intensity at points on the x-axis having x > > d.

I am little weak in this topic

I tried as

For destructive interfernce path difference should be $\lambda /2$

Let the destructive point would be at a distance x from origin , then

The path covered by each source would be x+2d , x+d and x.