The best version of this you can find is the one on laserdisc. It's called the Godfather Legacy I believe, and it combines the first two, edited in for scenes of both movies to run in chronological order, though I can't remember if it added the third one on at that point or if there was a later, separate release that added the third one in when it came out.
The only thing I'm not sure about is whether this is the exact same edit used on televisions, because I'm pretty sure there's no editing in the sense of any of the violence or language used on the laserdisc copy and I'm presuming they had to cut some of that out for the tv version.
While it's not dvd, and I really wouldn't expect a dvd or blu-ray or any other future medium version, the laserdisc can still be found occasionally on ebay and laserdisc players are supercheap now. If you're a bit of a cinephile you can find the best versions available of a number of films that haven't made their way to blu-ray, and they'll almost always be of better quality than their VHS tape counterparts.