Combine olive oil, 3 tablespoons lemon juice, 3 cloves minced garlic, 2 teaspoons oregano, salt, and pepper in a small bowl. Place the cut-up chicken in re-sealable plastic bag. Place the zucchini, summer squash, bell pepper and onion in another re-sealable plastic bag. Add 2 tablespoons of the marinade to the vegetables and toss to coat. Pour the remaining marinade over the chicken and toss to coat. Refrigerate the chicken and vegetables for 1 to 4 hours to marinate. Combine the sour cream, cucumber, dill, 1 teaspoon lemon juice, 1 minced garlic clove, and ¼ teaspoon oregano in a small bowl. Cover and refrigerate until ready to serve. When ready to cook, heat the grill. Alternately thread the chicken and vegetables onto 4 skewers (pre-soaked in water for 30 minutes if using bamboo skewers). Grilled, covered, over medium heat for 6 to 9 minutes or until the chicken is no longer pink in the center and the vegetables are crisp-tender, turning occasionally. Serve with the Tzatziki sauce.