Rub corn, onion, bell pepper, and jalapeños lightly all over with olive oil. Place on a barbecue grill over a solid bed of hot coals or high heat on a gas grill (you can hold your hand at grill level only 2 to 3 seconds); close lid on gas grill. Cook, turning corn occasionally and other vegetables once, until slightly charred all over, about 5 minutes total for jalapeños, 8 to 10 minutes for other vegetables. Remove to a board as done; let cool. Cut corn kernels from cobs, chop onion and bell pepper, and mince jalapeños.
Meanwhile, in a bowl, combine tomatoes, garlic, cilantro, lime juice, and cumin. Stir in grilled vegetables and add salt to taste.
Nutritional analysis per 1/4 cup.