A monochromatic light source of wavelength  $\lambda$ is placed at S. Three slits  $S_1, S_2 and S_3$ and   are equidistant from the source S and the point P on the screen.  $S_{1}P-S_{2}P=\lambda /6$ and  $S_{1}P-S_{3}P=2\lambda /3$. If I be the intensity at P when only one slit is open, how many times the intensity at P is increased when all the three slits are open