Instead of having the Zip Codes as three different columns in your data, put them in one single column with another column that labels them. The following links provide further information on best practices for setting up your data as well as a reshaper tool that will make changing your data easier:
Hope this helps!
Thanks for the quick response, Tracy! If I follow your suggestion and make one column for location, I think I will lose the relationships between project, engineer, and salesperson that I would like to use for filtering. For instance, I'd like to be able to click on a project mark and have the viz filter to a map showing three marks - one for the project location, one for the engineer location (for the project in question), and one for the salesperson location (again, for the project in question).