Im guesssing that by "in dmc" you mean that you are using the Data Management Console with a DataMigrator server and want to create a Data Flow with with a SQL SELECT statement. You can do that like this.
SELECT
C1 ,
MAX(IF (C2 = 1 , C3 , '' ))|| MAX(IF (C2 = 2 , C3 , '' ))|| MAX(IF (C2 = 3 , C3 , '' )) AS C3
FROM
c123 T1
GROUP BY
C1