As a rule of thumb, if you see someone with a piece of gear that has the same Main as all three Subs, a.k.a. a pure roll, it's either been save scummed, gear edited or ordered, directly or indirectly, from someone who used a method like that. Like, personally, I don't use any methods unintended by the game to get Sub abilities but I still have a handful of very unlikely pieces of gear because they're very commonly seen at middle-to-high levels of play, such as

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, and so are easy to order from Spyke.
Otherwise, gear like

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is most likely to be a 'natural roll': broadly speaking, the sub ability represented is 5× more likely to appear when levelling the gear up or rerolling it via Spyke, and you've got a 1/36 chance of getting a natural roll each time you try, meaning that you'll probably end up with a couple of them just from levelling up gear even before you get into rerolling much. If you want to know what ability a piece of gear is likely to roll, consult
this page.
Crazy stacking of gear abilities (like 3 Mains and 3 Subs of

) that isn't possible via natural rolls never gives
very significant advantages in battles and often is inferior to using a greater variety. As Judd will tell you, the more you use a single ability, the less the increases in its effects, and there are even hard caps in place for

, so it's impossible to reduce the number of shots you need to splat an opponent using it (e.g. the Aerospray starts at 24.5 damage per shot and will very quickly max out at 24.9, so it always takes at least 5 hits).
So it's possible to create most useful sets with natural rolls alone, and you don't even strictly need those - you have half of your abilities in Mains alone. If I were to name one significant exception, it'd be

, where it's really useful to have at least one pure roll to get the maximum effect.