ISO (sensor sensitivity) is arguably the least influential of the three. Whereas shutter speed and aperture have stylistic reasons for changing them, changing your ISO (sensor sensitivity) is only done when you don't have any room to change the other two values. And increasing your ISO only has negative effects on the quality of your images, as a higher ISO sensitivity also leads to more graininess and noise in the image. You want the ISO as low as possible to avoid this, but sometimes you have to sacrifice graininess to bring in more light (caveat: there could be times where you want a grainy or noisy image if you like the effect and/or for the sake of artistry).