T-SQL (SS2K8) :: Update A Field With Condition

Table A - data example
Idno Entity Flag
1 1 y
2 1 n
3 1 y
4 1 y

Table B - data example
Idno Entity
1 1
2 1
3 1

Table C - data example
Idno Entity
4 1

I want to update the value flag in table A.Update flag from n to y if idno A in table B and and idno A not in table C, so from table A above, first record and 3rd record remain unchange but 2nd record, flag will be changed from n to y. 4th record where idno A not in table b and in Table C so flag will change from y to n. is it possible, or do i need to use the "fetch" method to compare record by record.