maybe something along these lines.....???
WITH cte AS
(
SELECT CID,
CDate,
Dept,
row_number() OVER (PARTITION BY cid,dept ORDER BY cid,cdate,dept) rn
FROM #tTable
)
SELECT CID,Cdate,Dept
FROM cte
WHERE rn = 3 AND dept = 'C'
ORDER by cid,cdate
edit...simplified
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