Heat gas or charcoal grill. Carefully brush oil on grill rack. Place tomatoes, cut sides up, and mushroom caps on grill over medium-high heat. Cover grill; cook tomatoes 4 to 5 minutes, mushrooms 6 to 8 minutes, turning once, until tender. Cool; peel tomatoes and chop. Chop mushrooms; set aside. Cut bell peppers lengthwise into quarters; remove stems and seeds. Place peppers, skin sides down, on grill over medium heat. Cover grill; cook 10 minutes or until skin is blackened. Place peppers in bowl; cover tightly. Cool 5 minutes. Peel off blackened skin; rinse. Chop peppers; set aside.