The tri-beam laser rifle has the appearance of a "sawed-off" laser rifle, missing the tube on the bottom length of the barrel, being rather shorter than other models, and having a new emitter aperture and attached wires. The new emitter fires three laser beams with each shot. It is the main weapon of the New VegasBrotherhood of Steel. It uses 3 microfusion cells when fired, causing it to have an effective magazine size of 8. This weapon also degrades very quickly and is very expensive to repair. With the Jury Rigging perk it can be repaired by normal laser rifles. The weapon is unable to be modded.
Due to the fact that it fires 3 beams at once, it can score a critical hit for each beam outside of V.A.T.S. This effect allows it to sometimes, (with a critical hit for each beam) hit a huge amount of damage (132) per shot. Depending on your perks and Luck, this weapon can be a good choice just for its 3 beam shot.
Hidden Valley bunker - given by Knight Torres after helping her find a missing laser pistol. (The tri-beam laser rifle given by Knight may be bugged.)
Silver Rush - one can be found on the counter, in poor condition.
Another may be carried depending upon your level with the execution squad in I Could Make You Care.
Farther in the game, when you find Great Khans in the Mojave Wasteland they will sometimes carry the tri-beam laser rifle.
In the area where you get the alien blaster, 2 of the 3 aliens are equipped with this weapon. (Requires "Wild Wasteland" trait, otherwise it will be some mercenaries instead of aliens.)
Several Brotherhood of Steel members carry this weapon.