'Cattivo' in the modern sense comes from a specific expression, captivus diaboli ("prisoner of Satan"), and it means straight up "evil/bad/of bad quality". By itself, the older meaning is closer to 'wretched/worthless'. And the other occurrence of cativo in Fiore is lassa la sua daga cativa e vole la tua bona, so literally "he leaves his useless dagger and wants your good one".
The three guys seem to be looking for renown (i.e., that one of them would become the guy who wounded/defeated the master) and they are the ones talking, and trying to pump themselves up, while at the same time knowing deep inside that they are taking a big risk (which would explain the shift from 'it will be a miracle if he gets away alive' to 'may God make him miserable'). First thing the master does, is dashing their artificial confidence: they are 1) inferior fencing material and 2) have very little knowledge. Then he appears to be goading them into either giving up ("come who knows or can", i.e., since you deep down you ) or to prove they actually have some skill by attacking him one at a time (a uno a uno). Which is why he then says that even if they were one hundred, he would still not take a single wound: i.e., his guard would block any kind of technique (point, edge, throwing). It is true that he subsequently uses the plural (e cum quelo passar io me covro rebatendo le spade, ve trovo discoverti, e de ferire ve faro certi), but it is also a forced choice, sintactically speaking, in order not to make the period a nightmare, and it does not necessarily mean that he's referring to the three attacking him at the same time (which would make the 'one hundred' reference a ridiculous boasting, in case they were meant to attack all at once, and would also defeat the 'prove me wrong about you not having any skill by attacking me one at a time'). I have the impression that the episode is more about "no matter what technique is used, either lunging, slashing or throwing, I won't be wounded" rather than "no matter the number of attackers I will fuck them all up".
Firstly, I appreciate the nuanced response.
Some contentions: Fiore saying they're inferior fencing material and then that they have very little knowledge, seems sort of redundant if he's talking purely in terms of material quality, rather than material and moral, since it's because of their lack of knowledge that they're inferior fencing material. He tells them to "come at me bro" one by one, which doesn't necessarily mean they're going to sit tight and wait their turn rather than just trickling in at him in fairly rapid succession. All three defensive actions that Fiore performs could span the course of a minute or so, if these guys are as trash as he says. With regard to his boast about taking on 100, I thought that part was just him talking mad shit. Three at once I'd consider the limit, and even then the defender will probably receive mortal wounds (unless he really is a master).
But mostly I look at the art and the main issue I have is that if this is indeed just about "no matter the technique used, I'll beat it with my own technique" there's already dozens of other images depicting 1v1 fighting in the manuscripts. The same guard could be conveyed in any of those rather than a random pic of a "3v1 kinda, sorta, not really." Splitting those images up, few that they are, would not have broken the page space bank, so I choose to assume that these are examples of him implying he can go up against multiple foes when pressed.
Anyway, this is way off in the weeds now. If I'm wrong about what I'm seeing in Fiore, mea culpa, however he is not the only source that we could use. A German source MS 3227a (Other Masters Section) apparently talks at length about fighting 4-6 peasants from the Iron Gate guard. The translation is by Hans Stoeppler.
Full transcript here, keyword search for Iron Gate. It should be noted that previous advice in the manuscript tells you to exercise prudence and avoid doing it, but regardless it exists as an addendum.