The Laser 3Dot is described as:
Uses three lasers for fast target acquisition
It can be found north of central Montuyoc at an air strip. A “medication air drop” side mission is nearby. The accessory case is on a shelf of a metal rack outside, between the buildings.
There are a bunch of bad guys nearby guarding the plane for the side mission, but there’s also a sniper tower at the nearby road intersection and checkpoint which you should also be mindful of.
I had just come from the mine and stolen an APC which is the perfect vehicle for just hooning in here and smashing the place up, making you practically impervious to small arms fire. That’s fine if you don’t mind risking the plane, otherwise I suggest something a bit more tactical maybe using the cover of night and a distraction from your rebel support skills or a flare before using sync shots to start picking guys off.
The three lasers are meant to improve accuracy, but I do find them slightly distracting. I guess it makes you look a bit like the Predator though, which could be cool. However for handguns I do prefer the regular ATPIAL Laser Sight (found in Barvechos) which provides a single laser dot which I personally get on with better.
Perhaps try both and let me know in the comments which one you like or whether you use either at all.