The U-3 is one of the experiments carried out by the research team at the Island. It is a horrifying amalgamation of many beings all fused imperfectly into one. While it has the upper part of a human torso and a human head, its visage is monstrously warped and marked by strange scars upon its nape and crown, a clawed arm, and a tentacle in place of the other one. Below this, it is grafted in a centaur-like fashion into a deformed body, that has two front legs and two hind ones bonded by a centipede-like fleshy piece of muscle. Later, after receiving enough damage, the Plaga attached to its spine emerges to attack. The Plaga has evolved since its first stages and resembles a tentacle ending in a gaping mouth, lined with dozens of teeth. This tentacle also sports a heavy pair of pincers.
Osmund Saddler first mentioned the U-3 to Leon S. Kennedy, referring to it simply as "it". Later, in the underground cellars, Leon first fought the U-3 in a suspended set of crate rigs, in a battle where he continously dropped the rigs to the abyss below. When the last crate fell, the creature went down with it. Nevertheless, it returned for a second bout, during which its weakened Plaga erupted from its spine. After destroying the Plaga, the rest of the body went down with it.