-- T1
create table T1 (
    ID  integer not null
);

alter table T1
  add constraint PK_T1
      primary key ( ID );


-- T2
create table T2 (
    ID     integer not null,
    T1_ID  integer
);

alter table T2
  add constraint PK_T2
      primary key ( ID );

alter table T2
  add constraint FK_T2_T1
      foreign key ( T1_ID )
      references T1 ( ID );


-- T3
create table T3 (
    ID     integer not null,
    T1_ID  integer
);

alter table T3
  add constraint PK_T3
      primary key ( ID );

alter table T3
  add constraint FK_T3_T1
      foreign key ( T1_ID )
      references T1 ( ID );