For nth minima:
⇒bsinθ=nλ
(λ is very small, so sinθ is very small, hence sinθ≃tanθ.
⇒btanθ=nλ
⇒bDy=nλ
⇒yn=bnλD (Position of nth minima)

B→1st minima and A→3rd :
y3=b3λD,y1=bλD
⇒Δy=y3−y1=b2λD
⇒3×10−3=b2×6000×10−10×0.5
⇒b=3×10−32×6000×10−10×0.5
⇒b=2×10−4m