The picture to the left illustrates how you might stack cook. In the large bottom pan you could have a roast with some potatoes. In the middle pan you could have some broccoli and in the top pan you could have some corn.
When stack cooking always remember to start each individual pot on a burner until you hear the steam valve whistle. Put food that has to cook the longest in the bottom pot.
Heat is transferred from the bottom pot up to the top pot.