Martinez got his first in the 73rd minute to add to Bruno Zuculini's opener for the Buenos Aires outfit and launch the late goal fest.
Rafael Santos Borre converted a penalty in the 89th minute after he was felled in the box to make it 3-0 and Martinez added the gloss in stoppage time.
It was the the 25-year-old Argentine's final goal for River Plate before he leaves for Atlanta United in the MLS in the United States.
On Saturday night, holders Real Madrid will take on Al Ain in the final. The Spaniards are seeking an unprecedented third consecutive crown and a record fourth overall.