First, different last names doesnt eqaul split reservations automatically. My wife travels under her unmarried name and we are are always on the same record. We are both Gold and prefer to either both be upgraded or neither. Now, funny thing is they have called only one of us a couple of times saying it was the last seat so at the airport maybe things get split up no matter what.

I would say if you want the best chance to upgrade leave it split. It will be much more difficult to upgrade 3 especially as a silver. This way you may get one at least confirmed before the flight and then have a chance for a battlefield on the others.

The only thing of concern is that you may get split up on your seating with the seperate records.