I learned the 2-hole dirthole set (two holes, one trap/dirt pattern) from Charlie Dobbins when I was a kid.
Arguments over the most efficient set, or best trap, or best lure or location, are just that.
Whatever works for you, and whatever works for others, great. I've seen some sets that seemed very silly in terms of generating comments like, "gee...that takes a tremendous amount of additional effort" type of sets such as a big mound made with a 5-gallon bucket of dirt, etc. But if it works, cool.
Same with simpler sets that take minimal effort, and are just as successful. They are a great tool to have in the set arsenal, too.
Whatever works, great. I love trying new sets...it keeps this fun for me. There are a lot of people trying to learn new stuff on this site. Others are just looking to share some ideas.
I use both singles and doubles (holes at individual sets).
As for 3 dirthole sets in one locations (3 sets, 3 holes, 3 traps)...gang setting is something I've done (and still do) too. I like to use different baits and or lures at the three sets...to answer your original question. I don't like to take a chance on not offering the preferred menu item of choice for that critter on that day. Does it work better than if I only make the sets all the same food or lure? I don't know. I catch some fur if I offer the variety...so why not change it up a bit?
jim-NE