My son just called. There's enough reason to worry but they can't do nothing now. One of the twins is half the weight of the other and over two weeks behind in development. They're staying there over night and will come home tomorrow. Next Tuesday they have to go back for another check up. They have to prepare for lots of problems during the rest of the pregnancy. Nothing to be happy about but on the other hand not as bad (for now) as we expected.
The twins are boys and the third is a girl.