Rub corn, peppers and onion with 2 tablespoons of the olive oil and place on a white-hot grill. Grill until corn is lightly browned and peppers are charred. Remove charred skin from peppers (leaving some of the charred skin is fine) and corn kernels from cobs. Combine corn and peppers with remaining ingredients and let stand at room temperature for 1 hour to combine flavors.