Show jCarousel if number of items are more than 3
You can use JCarousel views as block display and then set a visibility condition by executing custom php condition test code (This will require to enable PHP Filter module)

$result = views_get_view_result('view-name', 'display_id');


Then you can analyze the $result and count the number of results.
Most probably you need to do ....

if(count($result)&lt;3)
    return FALSE; //Would not let to show that block
else
    return TRUE; // Would let to show