When I first tried Safari 3, it was unusable. Every time it loaded a page, it would fail to render and the previous page (or blank window) would remain. Only reloading would cause pages to render properly.
I tracked the problem down to SafariStand -- in particular, its site alteration feature (which was high on my list of suspects from the beginning). So if you want to use SafariStand with Safari 3, you'll have to switch the site alteration feature off.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20070614231242102