MERGE table1
USING (SELECT table3.keycolumn,
              table2.DataColumn1,
              table2.DataColumn2
       FROM table2
       INNER JOIN table3
           ON table2.anotherKey = table3.anotherKey
       WHERE table2.anotherKey = 'A1') tmpTable
ON 
   table1.keyColumn = tmpTable.keyColumn
WHEN MATCHED THEN
       UPDATE
       SET table1.DataColumn1 = tmpTable.DataColumn1
            ,table1.DataColumn2 = tmpTable.DataColumn2;