You can tell if a mod has a chance to roll triple stats before you pick it up. This is not all that useful if you're reset farming from the same campfire. If you don't loot a mod from a body, restarting your save will cause that monster to drop that mod again.
This only applies to "very rare" mods:
- All coils (weapon mods): 45-50%
- Melee/Ranged weaves: 20-??% (I've only found as high as 20 for these)
- Stealth weaves: 24-??% (24 is the highest I've found)
- Elemental weaves: 31-??% (I've only found as high as 33%)
- Elemental weaves are a bit weird compared to the others; I've found plenty of 31% mods that only have a single or double stat on them, whereas the others have always come with triple when they fall in their respective range.
- I'm not sure that coils can roll a triple combination of damage/handling/tear. It seems that at least one of the three has to be an element, but I don't know for sure. I've only found 36 mods with one of those three as the main stat; I could have just been unlucky. e.g. other people claim to have found xx/20/20 mods, but I've never found anything with xx/20/20 — only xx/20/11-13.
- Similarly, I haven't found any weaves with the triple combination of melee/ranged/stealth. One of the three has always been an element.
- Triple stat mods seem to drop about 5% of the time? Between opening hundreds of purple merchant boxes and looting large machines, this is the rate I've found. Merchant boxes and machines might have different rates, I'm not sure.
- Two mods with the same x% stat don't necessarily increase by the same amount. I think there's some kind of decimal system in place.
I hope this information is helpful to some of you. If anyone has anything to add or has any information that contradicts mine, please do share!