Is there any way to record video on an iPhone 3G?
Correct, and yes.

Here's one you can try: http://itunes.apple.com/us/app/record-video-for-free-iphone/id376582359?mt=8

You can probably find others by searching 'free video recorder iphone 3g'.

You will also find some extra options on Cydia if your phone is jailbroken.