How can I force visitors to view the slider in landscape mode?

You can't do this with Smart Slider 3, but with custom coding. You should hide the slider on portrait devices and display a message for the visitors to rotate their device to see the site / slider.

You can find helpful codes to achieve it here:
http://stackoverflow.com/questions/10975387/mobile-site-force-landscape-only-no-auto-rotate
http://stackoverflow.com/questions/8738072/forcing-web-site-to-show-in-landscape-mode-only
http://stackoverflow.com/questions/14360581/force-landscape-orientation-mode