SELECT
 events.b_g
FROM
 events
WHERE
 events.b_g NOT IN ( SELECT otf.b_g FROM otf )
AND
 events.b_g NOT IN ( SELECT requests.b_g FROM requests)

##</code><code>##

...
WHERE
 events.b_g IN ( SELECT otf.b_g FROM otf )
OR
 events.b_g IN ( SELECT requests.b_g FROM requests )
...

##</code><code>##

SELECT
 events.b_g,
 events.vector,
 SUM(tries) AS 'attempts',
 COUNT(DISTINCT events.observer) AS 'reporters'
FROM
 events

LEFT JOIN otf      ON events.b_g = otf.b_g
LEFT JOIN requests ON events.b_g = requests.b_g
WHERE
 requests.b_g IS NULL
AND
      otf.b_g IS NULL

GROUP BY events.b_g, events.vector
ORDER BY events.b_g, events.vector, 'attempts'

##</code><code>##

...
WHERE
 requests.b_g = events.b_g
OR
      otf.b_g = events.b_g
...