create or replace trigger t_trg
before update of column2, column3 on t
for each row
begin
  :new.column1 := :new.column1;
end;

Error report:
ORA-25006: cannot specify this column in UPDATE OF clause
25006. 00000 - "cannot specify this column in UPDATE OF clause"
*Cause: Attempt to create a trigger on update of a column whose
datatype is disallowed in the clause, such as LOB and
nested table.
*Action: Remove the UPDATE OF clause.