UPDATE trg
	SET col1 = ...
	FROM tbA trg
	LEFT JOIN tbB b ON b.tbAid = trg.id AND b.col2 = '...' 
	LEFT JOIN tbC c ON c.tbAid = trg.id AND c.col3 = '...' 

UPDATE tbA 
	SET col1 = ...
	FROM tbA alias_a
	LEFT JOIN tbB b ON b.tbAid = alias_a.id AND b.col2 = '...' 
	LEFT JOIN tbC c ON c.tbAid = alias_a.id AND c.col3 = '...' 
        where tbA.id = alias_a.id