styles unlimited value image fields
CSS. Your fieldset should be wrapped in one big div, with each field-item wrapped in it's own div. I see no reason you couldn't do this with CSS alone. 

Unless there's a specific reason you need each line to be exactly three images. In that case you would have to override the field template: https://drupal.org/node/1089656 

Then you would have to write some basic PHP logic that counts the number of field items and print those out in a 1X3 display.