Ho risolto così:

codice:
(SELECT id,website,category_name_it,nation,timestamp FROM tabella1)
 UNION 
(SELECT id,website,category_name_it,nation,timestamp FROM tabella2)
 UNION 
(SELECT id,website,category_name_it,nation,timestamp FROM tabella3) 
 ORDER BY timestamp DESC LIMIT 3
..e tutto sembra funzionare!!