Mix mayonnaise, sour cream, mustard and garlic powder. Spread a thin layer of mixture onto a large plate. Place chicken onto plate in a single layer, then spoon remaining mayo mixture over chicken. Cover loosely and chill for 2-4 hours.
Preheat oven to 375°F.
Combine last three ingredients and layer mixture over chicken.
Move chicken to a Pyrex dish large enough to fit the three chicken breasts in a single layer, keeping only the marinade that sticks to the chicken and discarding the remainder.