Grazie a tutti per il supporto. Ancora errore..osservate:

codice:
SELECT COUNT(id) FROM 
 (SELECT COUNT(id) FROM tabella1
    UNION 
  SELECT COUNT(id) FROM tabella2 
    UNION
  SELECT COUNT(id) FROM tabella3) 
AS t WHERE t.timestamp BETWEEN DATE_SUB(NOW() - INTERVAL 1 HOUR) AND NOW();

Errore riportato stavolta:

You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ') AND NOW()' at line 1


Santi subito!