I would think you can add a COUNT(*) OVER (PARTITION BY XXXXX) where XXXXX is what you include in your dense rank.
Then wrap this in a Common Table Expression and select where your new Count is = 1.
Something like this fiddler:
http://sqlfiddle.com/#!6/ae774/1
Code included here as well:
CREATE TABLE T
(
COL1 CHAR,
COL2 CHAR,
COL3 CHAR
);
INSERT INTO T
VALUES
('a','b','c'),
('a','s','r'),
('a','w','f'),
('b','b','c'),
('c','f','r'),
('c','q','d');
WITH CTE AS (
SELECT COL1 ,
COL2 ,
COL3,
DENSE_RANK() OVER (ORDER BY COL1) AS DR,
COUNT(*) OVER (PARTITION BY COL1) AS C
FROM dbo.T AS t
)
SELECT COL1, COL2, COL3, DR
FROM CTE
WHERE C = 1
Would return just the
b, b, c, 2
row from your test data.