Revamped version of the iPhone 3G with faster processing speeds. Repair of this device is similar to the 3G, and requires simple screwdrivers and prying tools. Model A1303 / 8, 16 or 32 GB capacity / black or white plastic back.
Yes they are interchangeable. But the contacts on the 3GS Flex are larger in size, so are a better design. If you want to replace a 3G or 3GS Home Button Flex get a 3GS Flex. If you already have a good 3G Flex then just use it on either iPhone.