Are you adding the criteria with Required = true? That would cause them to act as AND. On the other hand, if you add all criteria with Required=false it will probably match anything. I can't think of a way to require at least one. If you have other criteria as well so your query doesn't return too many results, you can filter the results to remove any hits that don't contain any of the property values. If not, I would do three different queries for the values and combine the results.

Edit: Now that I think about it I think it is enough to set Required to false. It won't include pages that don't match any of the criteria, so it should effectively work as OR.