Using 3 of the same mod is generally a bad idea because of diminishing returns, which applies for all mods across the board. I rarely even use x3 of the same for surges - only when both of my damage weapons match.

They stack but have greatly diminishing returns. I know 1 dexterity mod is a .8 multiplier and 2 only goes to .75

I assume you're referencing PvP? For me......there's zero considerstion in PvE. I never spend these mods in PvE.....only in PvP. The 3rd counts, but diminishing returns for each mod equipped....can't remember the numbers but you're better off with 2, not 3. I consider 2 to be all-in on a weapon. The 3rd is better spent elsewhere. If equipping 3 of these mods weigh against how often you'll engage and how.....with weapon 1 versus weapon 2 on a hypothetical 1 / 2 (of 3) mod split. I do this all of the time. As an aside...same goes for anti-flinch mods. Example - when saving loadouts, you may wish to save per class / subclass.....but you might also decide to save both a longer range and shorter range loadouts on the same subclass....but with different mod distribution. Does this make sense?

usually 2 are more than enough and adding a 3rd one barely has any effect.

Yes, it works That's what I use for my pvp builds. Duplicate loading mods on primary as well.

All work, they just have diminishing returns the more you have, I’d recommend 2x targeting for primary, 1x for secondary, 3x unflinching for primary (as much anti flinch as possible for primary duels) 👍