Returning rows where all non-NULL values are equal
The most compact syntax I can find is:

SELECT * 
FROM @T AS t
WHERE EXISTS 
(
    SELECT ISNULL(ISNULL(a, b), c) 
    INTERSECT 
    SELECT ISNULL(ISNULL(b, c), a) 
    INTERSECT 
    SELECT ISNULL(ISNULL(c, a), b)
);


Based on an idea from one of my old blog posts that describes how to use INTERSECT and EXCEPT to replace comparisons like a &lt;&gt; b OR (a IS NULL AND b IS NULL) with NOT EXISTS (a INTERSECT b).