Can I update to iOS 4.3 firmware on the iPhone 3G?
No. Apple is officially leaving the iPhone 3G behind to run iOS 4.2.1 firmware forever. No more updates for the iPhone 3G will be forthcoming.
If you haven't updated to iOS 4.2.1 on your iPhone 3G you can find iPhone 3G firmware files here. The iPhone 3GS and iPhone 4 (AT&T only) will support iOS 4.3.