Joining CP-1/CP-2, it first went critical on 15 May 1944, and was at first used in the experimental physics work of the Metallurgical Laboratory for the Manhattan Project.

CP-3 was initially fueled with natural uranium and used heavy water as a neutron moderator.

In January 1950, the reactor was dismantled due to suspicion of corrosion of the aluminum cladding that surrounded the control rods.

After the reactor was decommissioned, the fuel and heavy water were shipped to the Oak Ridge National Laboratory.

Pipes, valves, and building debris were placed in the reactor's containment shell, which was then filled with concrete.