CREATE TABLE #three
(
  colA varchar(32),
  colB varchar(32)
);

INSERT #three(colA, colB) VALUES
  ('abc', 'xyz'),
  ('def', NULL),
  (NULL,  'ghi'),
  (NULL,  NULL);

SELECT ColA, ColB,
  ColC = LTRIM(COALESCE(colA, '') + COALESCE(' '+colB,''))
FROM #three;

DROP TABLE #three;

ColA  ColB  ColC
----  ----  -------
abc   xyz   abc xyz
def   NULL  def
NULL  ghi   ghi
NULL  NULL