Thanks Roger Govier the problem has been solved but let me know i had try
your given formula also as follow
=IF(COUNTIF(Sheet1!A:A,Sheet3!A2),VLOOKUP(Sheet3!A 2,Sheet1!$A$2:$C$4,3,0),VLOOKUP(Sheet3!A2,Sheet2!$ A$2:$C$4,3,0))
and got good result, is this application correct because it is easy and will
not creat problem in future.

thanks in advance
Zafar