insert into t (f1, f2, f3)
  select v1, v2, v3
    from dual
   where not exists (select null
                       from t
                      where f1 = v1
                        and f2 = v2
                        and f3 = v3);

begin
  insert into t (f1, f2, f3) values (v1, v2, v3);
exception
  when dup_val_on_index then
    null;
  when others then
    raise;
end;