Puzzle can be solved in 4 moves:
  ****
  *bc*
  *a_*
  ****
Move piece 'c' down
  ****
  *b_*
  *ac*
  ****
Move piece 'b' right
  ****
  *_b*
  *ac*
  ****
Move piece 'a' up
  ****
  *ab*
  *_c*
  ****
Move piece 'c' left
  ****
  *ab*
  *c_*
  ****