The
Infinity Classics website has already done a fine job of organizing the hierarchy of the different models in each of the various lines of speakers that were manufactured from about 1968 through approximately 1993, so I'll only address the Reference Series of speakers here. I'll leave it to you to peruse this excellent website to learn more about the many other speakers that Infinity created.
According to Infinity Classics, the RS-IIIA/B was part of the Reference Series of speakers manufactured from about 1982 through 1984. There were 10 speakers in the line (unheard of today); the IIIA/B was 3rd from the top, with only the RS-IB and RS-IIA/B ahead of it. From this series of speakers, the 1B is unquestionably the "Holy Grail" but the IIA/B is no slouch either. However, the ultimate Holy Grail of Infinity speakers would easily have to be the various incarnations of the mega-expensive IRS, first introduced in 1980.
IMNSHO, the IIIB is a very fine example of the classic Infinity sound from this era when Infinity was clearly on the cutting edge of speaker design. It employs the same tweeter as the much more expensive models in the Reference Series, and only gives away the last octave or two at the bottom to the bigger and much more expensive IB and IIA/B. Infinity clearly did not ignore this speaker's development in favor of the bigger models, as they introduced a new dome midrange (the Polydome) that was used in the IIIB, 4B, 5B, and RS-6, and in the later Kappa series of speakers, instead of a more conventional cone midrange. The only way my IIIB's will ever leave my house is if I find a pair of IIA/B's or (if the wife would ever let me and I could afford both the speakers and the amps to drive them) some IB's.