How to set a trigger on delete multiple parent and child rows?
I don't see the need to use a trigger. You could use two DELETEs:

DELETE C
FROM Table_C C
INNER JOIN Table_B B
    ON C.B_ID = B.B_ID
WHERE B.A_ID = 1;

DELETE FROM Table_B 
WHERE A_Id = 1;


If you want to use a trigger, you can use this code inside:

DELETE C
FROM Table_C C
INNER JOIN DELETED B
    ON C.B_ID = B.B_ID;


Justa as a note, remember that if you have a foreign key in that table, you could configure it ON DELETE CASCADE (but be careful with your deletes)