Also, what would the initial 3 be about, if this was a serious question ?

While it was implied that Antonio Andolini was perhaps involved in criminal activity of some sort (GFII) along with his brother (The Sicilian), who is never named, it doesnt seem like there's enough action there to create three full films.

Unless of course you're talking about the first three episodes being in the space between the GFII DeNiro parts and GFI, from about 1927 to 1941/1945, which I would love, instead of all three taking place before the young Vito Sicily scenes, which would mean it would all be 19th century stuff.

Either way, probably not likely, like plawrence said.