(Covers information from the S.D. Perry novels)
The Trisquads were utilized by the Caliban Cove Facility for security, a job the somewhat mindless B.O.W.s took to very well. Trained, or more accurately, programmed to only engage in combat only when another team was incapacitated. Their combat ineffectiveness was ultimately their undoing, as David Trapp's S.T.A.R.S. team managed to flank and destroy an entire squad without it even returning fire.
The entire Trisquad B.O.W. series was ultimately wiped out by the S.T.A.R.S. Exeter Branch.
Development and Traits Edit
Trisquads were developed using a special variant of the t-virus created by Doctor Nicolas Griffith, which created infected that were extremely impressionable and resistant to the virus's normal cannibalistic behavior. Trisquads were put through a series of vigorous (for a zombie) tests, often involving picture puzzles or riddles. While the rewards for these were probably something to ease the B.O.W.'s painful mutations, they were co-opted by a sympathetic Umbrella researcher in order to assist the S.T.A.R.S. team to shut the facility down. They opted in groups of three (hence, Trisquad), and there were at least four teams present at the Caliban Cove facility.
The Trisquads were all armed with M-16 assault rifles (variant unknown, though likely the A1, as it was capable of fully-automatic fire). How well trained the B.O.W.s were with these weapons is unknown, though they seemed to have focused more on volume of fire rather than accuracy, and were ultimately outfought by the Exeter S.T.A.R.S. with their Berettas.
The Trisquads weren't especially combat effective, and would only engage a hostile when another team became incapacitated. This left them extremely vulnerable to flanking movements, and they were generally outclassed by any thinking opponent. Likely, they would've been deployed in support of heavier Umbrella units, such as T-103 Tyrants or 3K Hunters.
The Trisquads, despite not being tempted to take bites out of their opponents, were still vectors for the t-virus, and Rebecca forbade any of her teammates from using the weapons they were armed with. Karen Driver was infected upon touching a gurney covered in dried blood, presumed to have been a Trisquad's blood, as they were the only human bio-weapons on the island. Interestingly, Karen mutated from the dried blood into a normal zombie that immediately attacked an unflinching Trisquad that stood idly as she devoured part of its face.
Whether or not Umbrella picked up the B.O.W. for use further development elsewhere is unknown. There were no Trisquads encountered in the Envirotest A facility or referenced in any other Umbrella research, so it seems highly unlikely.