Evaluate Multiple In One Statement

Question

How would one evaluate 3 fields to determine if they are blank?  I am creating an intranet page for my company, and the login screen requires the users 1st name, users last name and their company id.  What I am needing to do is if all 3 fields are blank, highlight (and by highlight I mean backcolor) all 3 textboxes red (txtFirstName, txtLastName, txtComID), if 1st Name is entered BUT the other 2 fields remain null, then only highlight txtLastName and txtComID Red.  What is the way to evaluate in C# to check all 3 fields and only highlight RED the field's that are null?