Anyone know how to write the logic for a workflow that I'm working on? I'm trying to keep new inventory items from being saved without any of the three price level fields being blank or $0. I know the fields needed are the following:

{pricing.pricelevel.id}=18
{pricing.pricelevel.id}=20
{pricing.pricelevel.id}=21

I thought about CASE WHEN but not sure how to finish out the logic. Any help will be greatly appreciated.