[Zope] How do you check for null search fields?

I have a search form with 3 different fields and need to verify that there is a value in at least one of them. I know that <dtml-if expr="some_field"> will test for one, but how do I determine if anyone of the three contains a value? I've checked the how-to's and Google but can't find anything. I'm using ZODB. TIA, Michael