Alexander Alfaro was found guilty on 16 of 17 counts in the execution-style slayings.
Six men and boys have been charged in the attacks, while three have now been convicted.
The victims were shot in the back of the head as they knelt in front of a wall, and one was slashed with a machete.
The fourth victim in the attack has testified at the three trials.
Alfaro's half-brother and another man are already serving multiple life sentences for the killings. Three defendants' cases are pending.
Harvey, Iofemi Hightower and Terrance "T.J." Aeriel were friends from Newark who were already attending or planning to attend Delaware State University in the fall.
The jury reached its verdict after about 20 hours of deliberations over four days. As jurors filed out of court, several nodded toward family members in the gallery.
"I was in seventh heaven when the verdict came," said Shalga Hightower, Iofemi's mother. "I wanted it so badly for her, knowing that this one was the one that did that to her."
Alfaro admitted to police after his capture that he slashed Hightower with the machete before she and the others were shot. But on the witness stand, he denied doing it and said he was pressured by a detective to change his statement to conform to what other defendants had already told police.