Prepare barbecue (high heat). Brush bell peppers, corn, and sweet onion with some olive oil. Grill vegetables until well charred in spots, turning occasionally with tongs and removing pieces as they brown, 10 to 15 minutes. Cool slightly. Cut bell peppers and onion into 1/3-inch pieces. Cut corn off cob.
Heat 2 tablespoons oil in heavy small skillet over medium-high heat. Add garlic and cumin; sauté until garlic begins to sizzle but does not brown, about 30 seconds. Pour into large bowl; mix in lime juice and hot sauce. Mix in vegetables. Season to taste with salt and pepper. Cool completely; mix in chopped green onion and cilantro. Let sit in refrigerator for at least 2 hours.