SQL & PL/SQL

Hi,

I'm using oracle 12c. I have a trigger TRG1 before insert or update on table T1 and a function F1 to update the table T1. I'm calling the function F1 from Trigger TRG1. i.e

Trigger code:
CREATE OR REPLACE TRIGGER bfo_ins_upd_t1 BEFORE INSERT OR UPDATE ON T1 FOR EACH ROW
…..
IF inserting OR updating THEN
  F1(:oldvalue1,:oldvalue2,:newvalue1);
END IF;
Function:
CREATE OR REPLACE FUNCTION F1(val1 in varchar2,val2 in varchar2,val3 in varchar2)
) RETURN CHAR IS
BEGIN
    update T1 set col1='A' where col2=val1 and col2=val2 and col3=val3;
END;

I'm trying to test in plsql developer ide for updating the value in T1. After I click the post change button. i'm getting this error Table T1 is mutating trigger/function may not see it encountered in function f1.

Thanks