create table t1_test(g_id number, t_id number, r_id number, flag_1 number)

create table t2_test(r_id number, flag_1 number)

create table t3_test(r_id number, flag_1 number)

insert into t1_test 
values(10, 20, 1, null)

insert into t2_test 
values(1, 2)

insert into t3_test 
values(2, 2)

commit

update t1_test t1 
set flag_1 = (select nvl(t3.flag_1, t2.flag_1) from t2_test t2, t3_test t3 
              where t2.r_id = t1.r_id 
              and t3.r_id(+) = t1.r_id) 
where t1.g_id = 10

select * from t1_test

rollback

update t1_test t1 
set flag_1 = (select nvl(t3.flag_1, t2.flag_1) from t2_test t2, t3_test t3, t1_test t4 
              where t4.g_id = t1.g_id 
              and t4.t_id = t1.t_id 
              and t2.r_id = t4.r_id 
              and t3.r_id(+) = t4.r_id) 
where t1.g_id = 10

select * from t1_test