The 3GS has quite a weak benchmark as compared to the iPhone 4, and especially iPhone 4S.
Most games on the App Store don't cater to the 3GS users anymore.
<img src="http://cdn.gottabemobile.com/wp-content/uploads/browser.jpg" border="0" />
<img src="http://cdn.gottabemobile.com/wp-content/uploads/java.jpg" border="0" />
<img src="http://cdn.gottabemobile.com/wp-content/uploads/ios51.jpg" border="0" />
Though if you really do manage to get a game working on the 3GS, it should have no problem at all running on a newer hardware.