Please try out this custom JavaScript verification:
// Settings
var minimumResponses = 3;
var message = 'Too few responses.';
// Run
var numberOfResponses = 0;
$('input[name="hid_list_[% QuestionName() %]"]').val().split(',').forEach(function(item){
if (SSI_GetValue('[% QuestionName() %]_' + item)) {
numberOfResponses++;
}
});
if (numberOfResponses < minimumResponses) {
strErrorMessage = message;
}