Je suppose que les "trois premiers" sont les trois éléments les plus fréquents dans la colonne Expertise, mais cela peut être modifié si ce n'est pas ce que vous vouliez dire.
WITH T1 AS (
SELECT TOP(3) Expertise, COUNT(*) AS cnt
FROM Expertise
GROUP BY Expertise
ORDER BY cnt DESC
), T2 AS (
SELECT ROW_NUMBER() OVER (ORDER BY cnt DESC) AS rn, Expertise
FROM T1
)
SELECT (SELECT Expertise FROM T2 WHERE rn = 1) + ',' +
(SELECT Expertise FROM T2 WHERE rn = 2) + ',' +
(SELECT Expertise FROM T2 WHERE rn = 3)
Résultat :
dfdf,.NEt,fgfg