Is there a way to activate or enable HDR on iPhone 3GS running on iOS 5.0.1?
Of course. The easiest way would be jailbreaking your iPhone 3GS first, and then search for Springtomize 2 on Cydia. It’s available under BigBoss repo.
Once installed, go to Settings > Springtomize > Capabilities and enable Extra Capabilities and HDR.
There’s also iOS 4.x version of Springtomize but it’s highly recommended that you update to iOS 5.0.1 to enjoy many of its new features.