Related Listings shows 4 instead of 3

Above resoluation 1600 my related listings carousel shows 3 listings at time, which is what i want.

Below this resolution it then displays 4 listings which is too many for this resolution.

I have checked in file: class_shortcodes.php and ‘perrow’ is set to 3. How can I stop it from displaying 4 in other resolutions?

Is there another file where this needs changing?

Thanks in advanced.