input: from typing import List
  
  def combine_vectors(vectors: List[Vector3D]) -> Vector3D:
      """ Combines a list of 3D vectors into a single vector by summing their x, y, and z components.
      If the list is empty, the function should return a vector with components (0, 0, 0).
      >>> v1 = Vector3D(1, 2, 3)
      >>> v2 = Vector3D(4, 5, 6)
      >>> combine_vectors([v1, v2])
      (5, 7, 9)
      >>> combine_vectors([])
      (0, 0, 0)
      """
output:     if not vectors:
          return Vector3D(0, 0, 0)
      
      x_sum = sum(v.x for v in vectors)
      y_sum = sum(v.y for v in vectors)
      z_sum = sum(v.z for v in vectors)
      
      return Vector3D(x_sum, y_sum, z_sum)