Peter Parker wrote:
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.
Desperate Distribution Methods of the 1970s: (aka, pre-Direct Market Comic Book Fails)

Peter, you were right the first time. The "random" bags of three would always feature the same three comics in rotation. For example, there would be a Daredevil, a John Carter and a Hulk in one bag, then a Captain America, an Avengers and an Invaders in another bag. They wouldn't be all mixed up. There would be no chance of buying duplicates, since the assortments of three were always the same. Same goes for the Whitman "Gold Key" bags, DC bags and whatever else they were circulating. And you could always bet that the middle comic was going to be the least popular character. That was where you'd usually find the Human Fly.

Other publishers tried weird means of distribution in those late 70s days: Charlton reprinted some of its books under the "Modern Comics" imprint and sold them at Woolworths. A very small number of Archies had dimestore "three bag" distribution. I don't think Harvey ever got into it (I could be wrong). All Harvey ever did in those days was do print horrible "pocket book" B&W reprints of comics with cut-up and re-pasted panels. DC did a handful of those too, but Marvel's Pocket Books reprinted the pages in color and in their original panel formation--just very very tiny.

Back to the 3-packs, Marvel and Gold Key books were the most prolific; DC never really made a dent in that little side industry, but yeah, there were some. I got three issues each of Superman and Action in those bags. DC Comics Presents appeared in those bags, as did Batman, Legion of Super-Heroes, Superfriends, Justice League and probably Wonder Woman too. I don't think they ever dipped into any characters who weren't on the A-list. And I think some of them were just two bags, not three.

And I kid you not--comics distribution had become such a desperate thing in the late 70s that they actually tried vending machines for comic books! I saw them in big cities; you'd put in 35 cents or whatever, pull the knob and out would pop a comic book. The comics were, again--very very random and--this was in 1978--some of the issues were five or more years old. Needless to say, these machines did not catch on, but oh, were they interesting. People don't believe me that there were comic book vending machines, but Grandpa was there and he bought an ancient issue of the Twilight Zone from one of them.

Way, WAY later, in the mid-90s, there was a really weird distribution method which sold, very, very random assortments of remainder comics from the Direct Market. These would be sold in packs of 5 or even 10 or more and distributed to Toys R Us, Walmart and other such outlets. They were really weird--some of them had just Marvel Comics, some were DC-only, and still more would pack in very random issues of Marvels, DCs, Valiants, Images and god knows what else. Some of the comics would be fairly recent, while others could have been up to ten years old. You never knew what you were going to get in those sets (and the damn things didn't sell very well anyway).