Place chicken in a plastic bag set in a deep bowl. For marinade combine oil, lemon juice, mustard, the 2 cloves garlic, oregano, and pepper. Pour marinade over chicken in bag. Seal bag; turn to coat chicken. Refrigerate for 3 to 24 hours, turning bag occasionally.
For sauce, combine the yogurt, cucumber, the 1 clove garlic, and dillweed in a small bowl. Cover and chill until serving time.
Heat a large nonstick skillet over medium-high heat. Add half of the chicken and marinade. Cook and stir for 2 to 3 minutes or until chicken is no longer pink; drain. Repeat with remaining chicken and marinade.
Meanwhile, wrap tortillas in foil. Heat in a 350 degree F oven for 10 minutes. To serve, place a scant 1/3 cup of the chicken and a generous tablespoon of sauce on each flour tortilla. Top with shredded lettuce and roll up.