Hummer 2007 H3 Owner's Manual page 392

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 393
for more information.
392
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.