Test SQL to create tables in https://dba.stackexchange.com/q/173743/77371.

test_merge_tables.sql

CREATE TABLE t1 (id, count_1)
AS
  VALUES
    (1,9),
    (2,4),
    (3,3);

CREATE TABLE t2 (id, count_2)
AS
  VALUES
    (1,2),
    (3,3);

CREATE TABLE t3 (id, count_3)
AS
  VALUES
    (1,1),
    (4,8);