All three have different uses and different .

Aperture
- by closing your aperture you're increasing depth of field and sacrificing light
- by opening your aperture you're decreasing depth of field, blurring the background and gathering more light

Shutter speed
- using faster speed you can freeze subject motion and camera shake (if any), but you're decreasing amount of light your sensor receives
- using longer speed you usually make scenes dreamy, with blurred motion and you're prone camera shake. You also increase amount of gathered light.

ISO
- by using higher iso you're increasing gain, which allows you to shoot with faster shutter speeds or smaller apertures, while sacrificing image quality (depends on the sensor, to some extent it's usually not visible) and increasing noise, because you're not getting more light, you're just amplifying the signal. It's similar to what happens to sound when you crank up the volume - it gets too distorted if you turn the knob too much

Let's say you want to shoot with high shutter speed (e.g. 1/400) to freeze motion and have a fairly small aperture like f/16, but it's getting dark. You can only use higher ISO to match these requirements. To what extent this will be possible depends on the used sensor. Some are way too noisy at 800-1600, so that can limit your ability to capture the image you want. If you have a better low light performing sensor (like in some full frame camera), you can easily shoot at 3200-6400 ISO and still get acceptable image quality with barely any noise.

The best rule about ISO is usually this: shoot with the lowest possible ISO to get the image you want.