So you only want resulting rows from one of the rows in TableC, then you must add a condition for that.
WITH CTE AS (
SELECT TableA.KEY
,Max(TableC.Column1) AS Column1
FROM TableA
JOIN TableC ON TableA.Column_key = TableC.TableA_key
GROUP BY TableA.KEY
)
SELECT CTE.KEY
,TableB.Column1
,CTE.Column1
FROM CTE
JOIN TableB ON CTE.KEY = TableB.TableA_key