  Let A be a finite set of integers. We prove that if |A| is at least 2 and
|A+A| is 3|A|-3, then one of the following is true:
  1. A is a bi-arithmetic progression;
  2. A+A contains an arithmetic progression of length 2|A|-1;
  3. |A| is 6 and A is Freiman isomorphic to the set
{(0,0),(0,1),(0,2),(1,0),(1,1),(2,0)};
  4. A is Freiman isomorphic to a set in either the form of {0,2,...,2k} union
B union {n} for some non-negative integer k at most n/2 -2 or the form of {0}
union C union D union {n}, where n=2|A|-2, B is left dense in [2k,n-1], C is
right dense in [1,u] for some u in [4,n-6], D is left dense in [u+2,n-1], B,C,D
are anti-symmetric and additively minimal in the correspondent host intervals.
