Hummer 2006 H3 Owner's Manual page 340

When you have a flat tire, you will first need to set up
the wheel blocks before changing it. They are located in
the tool bag, in the swing-gate. See Removing the
Spare Tire and Tools on page 5-75 for more information.
5-74
To use the wheel blocks,
lift the wheel block as
shown and lock into place.
Then, use the following as a guide to assist you in the
placement of the wheel blocks.
The following information will tell you next how to use
the jack and change a tire.