(Covers information from the S.D. Perry novels)
3K Hunters were a species of Umbrella B.O.W. seen in Resident Evil: Underworld. Developed in the Envirotest A facility, they were nearing completion around the time of the Raccoon City, Pennsylvania viral outbreak in 1998. The B.O.Ws were a variant of the standard Hunters deployed both at the Mansion and in Raccoon City, distinguishing themselves from their brethren with the ability to become almost invisible.
3K Hunters were encountered when the survivors of S.T.A.R.S. Exeter Branch, Rebecca Chambers, Claire Redfield, and Leon S. Kennedy attacked the Umbrella facility known as the Planet. Leon and John became trapped in the facility and were forced to fight their way through wave after wave of BOWs in order to escape. They fought the 3K Hunters upon stumbling into their enclosure, ultimately defeating the monsters and escaping. Whether or not Umbrella developed more of these B.O.Ws is unknown.
Development and Traits
Not much is known about the 3K Hunter, or even where its name comes from. The B.O.W. was apparently designed and manufactured at the Planet facility in the American West. Judging by the design of their portion of the Planet (an urban block), these Hunters were developed primarily for urban combat.
In battle, 3K Hunters were just as fast, strong, and aggressive as their brethren, but had the added advantage of being able to turn almost completely invisible at will. The advantages to this are obvious, and it appears Umbrella intended to use them to ambush enemy troops fighting in urban centers. Whether 3K Hunters were considered effective enough to be marketed as viable B.O.Ws (alongside lickers and T-103s) is unknown, though possible.