Yeah, I really don’t know anything for sure.
After a quick look at the IMDb page, it says this:

The movie begins in 1979, with a brief flashback establishing the long and tragic history of criminal activity within, and by, the Corleone family.

I’ve only see Part 3 twice, so I don’t remember if there’s another flashback after the one that sums up the events of 1 & 2…

JEDIT: Just found this description on a youtube video of the flashback:

This alternate opening is not featured in any of the releases. It includes a montage of clips from the past two movies and a voice over by Al Pacino

Looks like it’s just on the AMC version, but was put together with Al’s VO when the film was originally made and it was decided to cut it.
Like I said, I’ve only seen Part 3 twice. Once on VHS, once years ago on AMC. That accounts for the confused memories.
It’s weird that AMC uses it though. They only show these movies as a marathon, so who needs the catching up? (I watched Parts 1 & 2 twice over the weekend, but skipped 3 both times)