Actualizar una sola tabla basada en datos de varias tablas SQL Server 2005,2008

Bastante sencillo:

UPDATE T1
SET result = t2.results
FROM [table one] T1
INNER JOIN [table three] t3
 on t1.reg_id = t3.reg_id
INNER JOIN [table two] T2
 on t2.venue = t3.venue