SELECT * FROM
  (
      SELECT to_char(co.created_at, 'YYYY-MM') AS month
           , COUNT(co.id) AS co_count
           , COUNT(tm.id) AS tm_count
        FROM classroom_observations co
       LEFT JOIN training_modules tm
          ON to_char(co.created_at, 'YYYY-MM') = to_char(tm.created_at, 'YYYY-MM')
       GROUP BY to_char(co.created_at, 'YYYY-MM'), to_char(tm.created_at, 'YYYY-MM')
      UNION
      SELECT to_char(tm.created_at, 'YYYY-MM') AS month
           , COUNT(co.id) AS co_count
           , COUNT(tm.id) AS tm_count
           FROM classroom_observations co
           RIGHT JOIN training_modules tm
          ON to_char(co.created_at, 'YYYY-MM') = to_char(tm.created_at, 'YYYY-MM')
       GROUP BY to_char(co.created_at, 'YYYY-MM'), to_char(tm.created_at, 'YYYY-MM')
      ) tbl
  ORDER BY Month;

month  | co_count | tm_count
---------+----------+----------
 2019-03 |        0 |        3
 2019-04 |        1 |        0
 2019-05 |        6 |        6

SELECT * FROM
  (
      SELECT to_char(co.created_at, 'YYYY-MM') AS month
           , COUNT(co.id) AS co_count
           , COUNT(tm.id) AS tm_count
           , COUNT(tw.id) AS tw_count
        FROM classroom_observations co
       LEFT JOIN training_modules tm
          ON to_char(co.created_at, 'YYYY-MM') = to_char(tm.created_at, 'YYYY-MM')
       LEFT JOIN teachers_workshops tw
          ON to_char(co.created_at, 'YYYY-MM') = to_char(tw.created_at, 'YYYY-MM')
       GROUP BY to_char(co.created_at, 'YYYY-MM'), to_char(tm.created_at, 'YYYY-MM') , to_char(tw.created_at, 'YYYY-MM')
      UNION
      SELECT to_char(tm.created_at, 'YYYY-MM') AS month
           , COUNT(co.id) AS co_count
           , COUNT(tm.id) AS tm_count
           , COUNT(tw.id) AS tw_count
           FROM classroom_observations co
           RIGHT JOIN training_modules tm
          ON to_char(co.created_at, 'YYYY-MM') = to_char(tm.created_at, 'YYYY-MM')
           RIGHT JOIN teachers_workshops tw
          ON to_char(co.created_at, 'YYYY-MM') = to_char(tw.created_at, 'YYYY-MM')
       GROUP BY to_char(co.created_at, 'YYYY-MM'), to_char(tm.created_at, 'YYYY-MM'), to_char(tw.created_at, 'YYYY-MM')
      UNION
      SELECT to_char(tm.created_at, 'YYYY-MM') AS month
           , COUNT(co.id) AS co_count
           , COUNT(tm.id) AS tm_count
           , COUNT(tw.id) AS tw_count
           FROM classroom_observations co
           RIGHT JOIN training_modules tm
          ON to_char(co.created_at, 'YYYY-MM') = to_char(tm.created_at, 'YYYY-MM')
           RIGHT JOIN teachers_workshops tw
          ON to_char(co.created_at, 'YYYY-MM') = to_char(tw.created_at, 'YYYY-MM')
       GROUP BY to_char(co.created_at, 'YYYY-MM'), to_char(tm.created_at, 'YYYY-MM'), to_char(tw.created_at, 'YYYY-MM')
      ) tbl
  ORDER BY Month;

month  | co_count | tm_count | tw_count
---------+----------+----------+----------
 2019-04 |        1 |        0 |        0
 2019-05 |        6 |        6 |        6