How do I adjust the Marzocchi Dirt Jumper 3 (mtb fork)?
Sorry to say, but those forks do not have much (if anything) you could adjust.

The dials your are seeing are usually associated with more 'base model' coil forks and you would use them to adjust the preload (SAG).

You would adjust the preload for forks to match the rider weight against the suggested preload value. XC preload is typically 15-20%. DH is more. Preload is the initial value by which the forks are depressed in a standing state. Think of it as the bit that extends when the wheel lifts off the ground (in short).

Anyway, regarding your forks. If those are indeed Marzocchi Dirt Jumper 3's, you would find this user manual much more useful. 

If those dials are still working (they typically seize up or break off on the inside) you can expect them to tighten the coil when turned in the '+' direction. However, experience have tough me that the difference in feel is negligible and in most cases those knobs are actually just for looks. You often find them on budget friendly bikes and often the dials have popped off.