The fewest number of fights that a fighter nowadays could potentially do this in would be 4 minimum. Their first fight (pro debut) to get them ranked by whichever major sanctioning body (WBA, WBC, IBF or WBO) for their first major world title, immediately move up to collect the second, and then immediately move up again to collect their third. This would enable a fighter to shatter the current record of 12 fights if that's what their ultimate goal is. We will see this current 12 fight all-time record broken and it'll be done at the lightest weights. Loma could've already done it in less than 12 fights. At one point he was even thinking of doing it in 10 fights or less. Take away a couple of his title defenses that weren't necessary to achieve this goal, particularly early on when he was at Featherweight defending against lesser comp at the time, and there you have it.
The other common metric used in determining who's the fastest fighter to win a major world title in however many divisions can be measured by how many days it took them from their pro debut to the date that the record was set, fewest number of days. When Loma tied Saensak Muangsurin's record that was set in 1975 for fewest number of fights to win a major world title, fewest number of days can and was used (at least on Muangsurin's Wiki page) to unofficially break the tie. Muangsurin technically did it 10 days faster than Loma. Using this second metric to break a tie can be applied here as well since the record for fewest number of fights is now shared between Loma & Tanaka. Loma accomplished this feat 3½ months faster than Tanaka did. He also won his first and second titles faster (fewer number of fights).
Obviously, the most important talking points here would mean actually contextualizing their runs for a far more meaningful comparison. It isn't needed here, however (it's Loma hands down). This would include but isn't limited to who they beat for each title and everyone in between. When did this happen in their opponents' respective careers, what were their opponents' records going into each fight and how much world level experience did they have, what were their current standings in the rankings (quality, were they P4P rated as well?), where did they defeat their opposition to collect these titles (on the road or at home?), and how did they beat them (by decision or stoppage, how dominant were these performances overall)? How many of these titles, if any, were vacant when they were won? Were they the A-side for these title fights, did they ever have to fight on their opponents' terms (lost a purse bid, coming off a loss, fight contested at a catchweight or at a weight less optimal for them), what other advantages were conceded on paper when the fights were made (size/reach/height, youth, speed, power, strength, stamina, durability, experience, style), etc.
