input: Write a function to generate a nested loop in Python that prints out the combination of elements from three given lists on a new line. The function should accept three lists as input and return a nested loop structure.
  assert nested_loop([1,2,3], [4,5,6], [7,8,9])=="""for x in [1,2,3]:
    for y in [4,5,6]:
        for z in [7,8,9]:
            print(x,y,z)"""
  assert nested_loop([10,20], [30,40], [50,60])=="""for x in [10,20]:
    for y in [30,40]:
        for z in [50,60]:
            print(x,y,z)"""
  assert nested_loop([], [], [])=="""for x in []:
    for y in []:
        for z in []:
            print(x,y,z)"""
  assert nested_loop([1], [2], [3])=="""for x in [1]:
    for y in [2]:
        for z in [3]:
            print(x,y,z)"""
output: def nested_loop(list1, list2, list3):
      return f"""for x in {list1}:
    for y in {list2}:
        for z in {list3}:
            print(x,y,z)"""