He was voiced by Drew Nelson.
In "For a Few Duncans More", Duncan, after making a clone of himself named Duncan 2, also made a second clone of himself named Duncan 3. Duncan tricked Duncan 3 into doing his schoolwork for him, while he and Duncan 2 played in the treehouse and had fun. While Duncan made this clone, Duncan 2 saw what he did and made a few clones of his own. When Duncan and Duncan 2 got back to Total Drama Daycare, they saw that Duncan 3 had gone rogue and and started making a mess of finger paint. Chef was scolding him for misbehaving and Duncan 3 splashed a bucket of paint in his face and laughed at him for it. Chef was so mad that he got ready to call Duncan's parents, so the real Duncan had to slip in, throw Duncan 3 away and apologized vehemently for his wrongdoings, getting himself out of trouble. Eventually, Duncan 3 teamed up with Duncan 2 and all the other Duncan clones to overthrow Duncan and take over Total Drama Daycare and later escape into the real world, where they caused chaos all over the place and didn't get in trouble for it.
Duncan 3 looks exactly like Duncan only his hair is completely raven.