Biggest thing I've ever tested only had about 50 seats, so I'm no authority on these big beasties (particularly this week when I've been using JAR-22 which has nothing to do with that end of the spectrum).
My opinion (and it's only that) is that the airworthiness department would carry out a probability analysis of the various double and treble failure cases. If any of these cases showed a probability significantly below the acceptable limit (probably 1 per 10E5 flying hours), it would need evaluation.
Thinking about FAR/JAR-25 which would be the relevant standards; the obvious double failure case is VMCL-2, but I seem also to recall a few mentions of required minimum handling qualities in the event of certain failure combinations in the flight control systems.
Any Airbus or Boeing FTE's lurking around here with something to say?
G
N.B. Jetnoise, if there are regulations they're almost certainly in JAR 25 or FAR-25. Both are available online, the former at
http://www.jaa.nl/jar/jar.html and the latter at
http://www.amerflyers.com/htm/rfars.htm