Try a DAX as below.
fmtCol = RIGHT ( "0" & INT ( TableName[Duration] / 3600 ), 2 )& ":"& RIGHT ( "0"& INT ( ( TableName[Duration] - INT (TableName[Duration] / 3600 ) * 3600 ) / 60 ), 2 )& ":"& RIGHT ( "0" & MOD (TableName[Duration], 3600 ), 2 )
Or deal with the format in query.
select Duration, convert(varchar(10),DATEADD(second,Duration,0),108) fmtSecs from t1