The evolution of the Vuelta from it's birth in 1935 onwards is highly interesting. It's been annual since 1955, but that editionm was the first since 1950 and there had been other breaks before that. There were plenty of good names in the 1955 start list, along with a high proportion of Spaniards who are names only in Wikipedia terms.
I don't think it's fair to dismiss the Vueltas of this period as just a warm-up for any top riders taking part. One thing the Vuelta organisers were consistently good at was attracting most of the big names at least once. We have Gimondi, Merckx, Maertens and Battaglin who won on their only participation, whilst other winners including Zoetemelk, Anquetil, Janssen and Pingeon entered twice. The list of champions through the 60s and 70s hardly makes it look like an unimportant race, even if that may be partly illusory.
In the 50s and early 60s the Vuelta was a two-week race and only gradually evolved into the three-week grand tour format. So the period in which there were three three-week races - Spain, Italy and France - in quick succession lasted no more that 30 years.
Geminiani and Nencini certainly look like guys who would have thrived on hard work and tough racing, but was either seriously attempting to win each grand tour he entered? Nencini died quite young but Geminiani has remained a cycling icon for decades and there must be a wealth of material - interviews and the like - out there to provide insights.
But apart from those two, it's safe to assume that no-one has ever tried to win all three in a season. The achievements of Froome in 2017/18 and Kuss last year suggest that nowadays it might not be a totally unrealistic proposition. I did wonder last year whether Vindigo was testing the water at the Vuelta, seeing how successful he could be should he ever want to enter again as an afterthought. It's not hard to imagine him winning Giro-Tour double one day, and if he does the Vuelta experience will be very useful.
Pogacar's still going for the Giro-Tour double this year, isn't he? I don't think many expect him to succeed, but the luck could go his way. He doesn't seem like one to back off any sort of challenge, so surely he'd try for the Vuelta if the treble's on - provided he's healthy of course.