The Laser 3Dot is described in the game as:

Uses three lasers for fast target acquisition

It’s located at the southern edge of Tabacal province, inside a Network Relay Station side mission. The accessory case is upstairs inside the station.

You must get into the station to reach the accessory case. Completing the side mission is not compulsory, but I have a separate video which explains how to do this easily.

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.