3947 was an automated personnel unit created by the Pralor, a race that no longer exists, in 2222.
In 2372, unit 3947 was found by B'Elanna Torres adrift in space on a disabled ship, and she repaired it. The unit then beamed Torres aboard one of its sister ships. He wanted Torres to build a module that could be used to build more models like himself.
At first Torres helps the units by building a prototype. However another ship with similiar units appeared and fire on the Pralor starship. These units were built by the Cravic. Torres learns that the units were used to fight a war between the Pralor and Cravic, and when both sides decided to call a truce, the units killed all the Pralor and Cravic in order to continue the war.
Torres destroyed the prototype before being beamed aboard Voyager. (VOY: "Prototype")