Let's say we have six options, six check boxes. The user must select three options to proceed, no more and no less. What happens when the user tries to select a fourth option? Should it even be possible? Should the non selected check boxes be disabled when three options has been selected? Or should the first selected one be deselected automatically when the user tries to select the fourth one?
Or is there a third or fourth alternative?
The solution is pretty simple.
When someone completes checking three checkboxes, disable all non-selected checkboxes. If someone clicks on a disabled checkbox, show an alert to the user that already 3 options are selected.
However, you can make the design more intuitive by having some progress element.
This is the state when the user has checked only two options. The submit button is disabled, checkboxes are active, progressbar is incomplete
User has checked three options. So button is enabled, other checkboxes are disabled and progressbar shows completed
