my iphone 3g does not have software update..... Go to Settings > General > Software Update

Correct, as that option requires iOS 5 or higher, and the iPhone 3G can not be upgraded beyond iOS 4.2.1

Sounds like you are not running a later version of iOS.

Under GENERAL/SETTINGS/ABOUT what do you see for your iOS version?

the 3G doesn't update past 4.2.1

No. It doesn't. That is only present if you are already running iOS 5 or higher.

Since the iPhone 3G can NOT run anything higher than 4.2.1, you will never see it on that phone.

The iPhone 3g can only go to iOS 4.2.1. The Software Update button was added in iOS 5 when updating over wireless networks and cellular was added. You cannot update except by connection to a computer running iTunes and only to iOS 4.2.1.