One of the many things that make Photoshop such a powerhouse when it comes to image manipulation is its ability to be extended through things like custom brushes and shapes, generated by users themselves.
Photoshop CS3 custom brushes are packaged as .abr files and to install them is actually pretty simple.
First, download the custom brush .abr file you wish to install (it may be zipped, in which case extract it first). Next, browse to the install path of your CS3 installation. Locate the Presets folder, and in that, the Brushes folder. The most common path under Windows is usually:
C:\Program Files\Adobe\Adobe Photoshop CS3\Presets\Brushes
Move your downloaded .abr file into that folder. Note that the file name determines the label under Photoshop, so it is a good idea to give the file a nice descriptive name, like “Polka Dot Brushes.abr” for example.
Now simply restart Photoshop, and you’ll find your new brushes available for use in the system.