merge clause with inner join
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;