reverse3

  Write a static method called reverse3 that takes an
   ArrayList of integer values as a parameter and that reverses each successive
   sequence of three values in the list.  For example, suppose that a variable
   called list stores the following sequence of values:

        [3, 8, 19, 42, 7, 26, 19, -8, 193, 204, 6, -4]

   and we make the following call:

        reverse3(list);

   Afterwards the list should store the following sequence of values:

        [19, 8, 3, 26, 7, 42, 193, -8, 19, -4, 6, 204]

   The first sequence of three values (3, 8, 19) has been reversed to be (19,
   8, 3).  The second sequence of three values (42, 7, 26) has been reversed to
   be (26, 7, 42).  And so on.  If the list has extra values that are not part
   of a sequence of three, those values are unchanged.  For example, if the
   list had instead stored:

        [3, 8, 19, 42, 7, 26, 19, -8, 193, 204, 6, -4, 99, 2]

   The result would have been:

        [19, 8, 3, 26, 7, 42, 193, -8, 19, -4, 6, 204, 99, 2]

   Notice that the values (99, 2) are unchanged in position because they were
   not part of a sequence of three values.  Recall that the primary methods for
   manipulating an ArrayList are:

	add(E value)		 appends value at end of list
	add(int index, E value)	 inserts given value at given index, shifting
				 subsequent values right
	clear()			 removes all elements of the list
	get(int index)		 returns the value at given index
	remove(int index)	 removes and returns value at given index,
				 shifting subsequent values left
	set(int index, E value)	 replaces value at given index with given value
	size()			 returns the number of elements in list