adding custom validation to side-by-side question

I have a side-by-side question which includes 14 "statements". The second column of the side-by-side has the heading, "Select and rank your top three challenges" which is asking for just that: of the 14 statements, rank the number 1, number 2, and number 3 challenges.
I've created a drop down list with only 3 options: Number one challenge, Number two challenge, and Number three challenge. Is there a custom validation I can add so that once "Number one challenge" has been applied to one of the 14 available choices, it is not available again? If so, can you explain how to do it? Currently a survey taker could say all 14 are their "Number one challenge" but we want each of the 3 available answer options to be used one time.