You won't be able to modify a dropdown based on the selection.
The easiest option it to do a Question Group for each category. Then, let each user decide if they want to nominate someone of skip.
1. CAT Committed
1.1 Do you want to nominate someone for the Committed category
YES/NO
If No, Go to 2, If Yes, add 1 to variable "nominations"
1.2 What's the name of your nominee?
1.3 In a sentence or two, explain how you think...
2. CAT Caring
2.1 Do you want to nominate someone for the Caring category
YES/NO
If No, Go to 3, If Yes, add 1 to variable nominations
2.2 What's the name of your nominee?
2.3 In a sentence or two, explain how you think...
[Add 4 more categories]
At the end, check value of variable "nomimations". If >= 3, validate form, otherwise, tell then need to vote at least 3, and loop back to 1.
An alternative is to have a Multiple selection question with the 6 categories at the beginning, and ask them to pick at the start the 3 categories they want to nominate. You assign a score of 1 to each reply. If the score is above or equal to 3, you move forward based on the categories selected, otherwise, you remind them they must select at least 3.
Then, the rest of the quizz is organized as indicated above, without the YES/NO question. For each category section, you display it only if the category was selected in the Multiple selection question at start.
