Thanks, all, for your comments. 

So, based on what was mentioned, there was no specific set of three issues packaged together, it could have been a random group of three, and depending on which titles were printed as the Diamond variants for each month.  I, too, had read that the long gap at the beginning of '78 was due to the huge demand in reprinting the first Star Wars issues, and they were the only reprints in the run.  As I had never seen the Diamond Amazing Spider-Man issues in the 180s, I wasn't aware the three-pack was Spidey exclusively. 

I had been long under the impression that they were a certain group of three for all bags, since all the ones I had seen at the time of purchase had the same selection.  But, the "all" I'm talking about may be two or three other copies on the same rack hook, as the drug store endcap I had purchased these from also hung the Whitman DC titles as well.  So, may assessment for 30+ years was flawed.

I have checked eBay from time to time, but everytime I do catch one of these bags up for auction it's either Star Wars or a Marvel bag that did not have a Diamond--those were new to me!

Like I said, everything I had previously read on the internet didn't seem to exactly ring true to my comics-buying experience with these variants when I was a kid.  Your comments so far have been very helpful, and I thank you for all your recollections.