I have the same problem here. Two 24″ NEC screens and a 30″ Dell U3011 which is connected via the fancy powered adapter.
I can only have two screens working at once. Eg after I upgraded to 10.9.3 the 30″ was black but both the NECs worked. Disconnected one of the NECs and the Dell worked.
Looks like no Apple developer uses more than two screens. And no-one in the public beta program either.
If you are a developer, file a bug report so that hopefully it will be fixed soon. That’s what I’m doing.