In small bowl, combine yogurt, mint, dill, 1/4 teaspoon cumin, one-third of garlic, and 1/4 teaspoon salt. Let stand. In pie plate, combine 1 tablespoon lemon juice, 3 teaspoons oil, oregano, 1/4 teaspoon salt, remaining 1/2 teaspoon cumin, and half of remaining garlic. Add chicken, turning to coat. Let stand. Prepare outdoor grill for direct grilling (or preheat grill pan) on medium-high. Grill chicken 10 to 12 minutes or until no longer pink in center, turning once. Meanwhile, cut cucumber and tomato into chunks; toss, in large bowl, with 1/8 teaspoon salt and remaining lemon juice, oil, and garlic. Transfer chicken to serving plates. Grill pitas until toasted, turning once. Serve with chicken, yogurt sauce, and cucumber mixture; garnish with dill.