Not a simple checkbox allow you to define that kind of condition in the prompt, but you can easily setup a display condition on the section(s) of your page showing your analysis only if 3 or more values are selected.

1) You create you prompt without any special setting

2) You create an analysis containing only the column of your prompt and set a filter as "is prompted"

3) Define a display condition on the section(s) of your page containing the analysis where the 3 minimum values are required and set the "True if Row Count" is >= 3

optional:

4) add a new section with the same analysis as condition but setting the "True if Row Count" < 3 and display a text message reminding your user at least 3 values are required