I would love to get more info here as well. In case anyone is wondering why I dragged up such an old post. Well I wanted to know how to tell the 3SGEs apart and did a search first.
The series one as far as I know has a two piece (sides) cam cover and kind of slopped cam covers at that. Then the series 2 and 3 have a rather flat and sort of thin cam cover.
The series four (BEAMS) is quite different and you can usually spot is straight away as it has BEAMS all over it. See Bill Sherwoods site for lots of pics.
But how to tell series 2 and 3 3SGEs apart I would love to know.