Reduction of choices available based on previously chosen VL choices

If my grammar in the title is anything like my calc syntax I'm in BIG trouble.

Please refer to the attachment. The plan is to let users, using FMP 9.x, enter then select their search criteria for use in a report; the Custom VL List for all three text fields (named Sort_Choice1, Sort_Choice2, and Sort_Choice3) contains the same labels as what you see on-screen: Date Range, School, Grade, Period, Teacher, Student, and Attendance Code.

What I want to do is prevent users from selecting a choice more than once. For example, if s/he picks Date Range for the primary sort and High School for the secondary sort, I don't want the goof to pick Date Range or High School again for the Tertiary choice. So, I don't know how to approach and solve the problem: Should I create a convoluted calc with test fields so as a choice is made it's eliminated from the available list choices, or...?

With script triggers this would be a little easier to put together but because it's v9.x, well...

TIA for your help and input!

Rich