ORA-04091: table USR.MYTAB1 is mutating, trigger/function may not see it

I have the following three tables,

myTab1:
----------
keycol1# number(10),
othercol1 number(10)
PRIMARY KEY(keycol1) ;

myTab2:
-------
keycol1# number(10),
keycol2# number(10),
othercol1# number(10),
othercol2# number(10),
PRIMARY KEY(keycol1# , keycol2# ),
FOREIGN KEY(keycol1#) REFERENCES Tabl(keycol1#) ON DELETE CASCADE DEFERRABLE) ;

myTab3:
-------
keycol1# number(10),
keycol2# number(10),
keycol3# number(10),
keycol4# number(10),
othercol1# number(10),
othercol2# number(10),
PRIMARY KEY(keycol1# , keycol2# ,keycol3# , keycol4#),
FOREIGN KEY(keycol1#) REFERENCES Tabl(keycol1#) ON DELETE CASCADE DEFERRABLE)

And, I have a trigger on myTab2, which has to populate the myTab3
whenever some insert/update/delete operation is done on myTab2.

The trigger looks like the following:
-------------------------------------
create or replace trigger myTrg1
after delete on myTab2
for each row
my_othercol2 myTab2.othercol2%type default 100;
begin
my_othercol2 := :old.othercol2 ;

update myTab3 set othercol2# = othercol2# + my_othercol2 ;
where keycol1# = :old.keycol1#
and othercol1# = :old.othercol1# ;
end ;

During deletion, I get the following. How do I avoid this?

ERROR at line 1:
ORA-04091: table USR.MYTAB3 is mutating, trigger/function may not see it
ORA-06512: at "USR.MYTRG1", line 7
ORA-04088: error during execution of trigger 'USR.MYTRG1',

Summary:
--------
myTab2, myTab3 are children to myTab1 and are connected by keycol1# as the key.

When I delete the keycol1# entry from myTab1, I want to delete the entry from both myTab2, myTab3. but, if I am deleting from myTab2, I want to keep the summarised info on the myTab3 table

Any simple way to do this, without disabling the constraints?.
Also, I prefer not to delete the child records then delete from the parents (unless thats the only way).

Keycol1, is the key which links many other tables, when I delete from myTab1, I am deleting from many other related tables, all connected by cascade rules


Thanks,