Nested "if" statement (I'm stuck)

If the value in cell A1 is found column B, then populate with the text "321", however if the value in cell A1 is found in column C, then populate with the text "121". If the contents of cell A1 are not in either columns B or C, then populate with "0" or "No". This is half of the statement (unable to nest successfully with the other half of the statement): =IF(ISNA(MATCH(A1,$B$1:$B$1000,0)),"","321"). Any help would be much appreciated.