Function to count Widgets in a given location and append the column classes to each.

functions.php

/* Split Individual Widgets into Columns */

function st_split_widgets($sidebar_position) {

  if ( is_active_sidebar( $sidebar_position )) {
		// count the active widgets to determine column sizes
		$the_widgets = wp_get_sidebars_widgets();
		$below_content_sidebars = $the_widgets[$sidebar_position];
		$number = count($below_content_sidebars);

		// default
		$split = "full-width";
		// if only one
		if ($number == "1") {
			$split = "fullwidth";
			dynamic_sidebar($sidebar_position);
			return;
		// if two, split in half
		} elseif ($number == "2") {
		$split = "half";
		// if three, divide in thirds
		} elseif ($number == "3") {
		$split = "third";
		// if four, split in fourths
		} elseif ($number == "4") {
		$split = "fourth";
		}

		// Turn on output buffering because we can't access individual widget objects
		ob_start();
		echo '<div id="aftercontent" class="'.$split.'">';
		dynamic_sidebar($sidebar_position);
		echo '<div class="clear"></div></div>';
		$sidebar = ob_get_contents();
		ob_end_clean();

		// search the output for the widget-container class
		$pattern = '/widget-container/';
		// define replacement for columns
		$replacement = 'widget-container one_'.$split;
		// widget content
		$content = preg_replace($pattern, $replacement, $sidebar, -1 );

		// define search string
		$search = "widget-container one_".$split;
		// define replacement for .last class
		$replace = "widget-container one_".$split." last";
		// find the last instance and add the replace class
		$pos = strrpos($content, $search);
		if($pos !== false) {
		    $content = substr_replace($content, $replace, $pos, strlen($search));
		}
			echo $content;
	} //endif
} //@end st_split_widgets()


// Assign the function to st_before_content
function st_before_content_widget() {
	do_action('st_before_content_widget','above-content-widget');
}
add_action('st_before_content_widget','st_split_widgets',10,1);
add_action('st_before_content','st_before_content_widget',1);

// Assign the function to st_after_content
function st_after_content_widget() {
	do_action('st_after_content_widget','below-content-widget');
}
add_action('st_after_content_widget','st_split_widgets',10,1);
add_action('st_after_content','st_after_content_widget',1);

style.css

/* Add to style.css */
#aftercontent {margin-top: 20px;}

#aftercontent.half .widget-container.S1,
#aftercontent.half .widget-container.S2,
#aftercontent.half .widget-container.S3 {width: 45.6%;}

#aftercontent.third .widget-container.S1,
#aftercontent.third .widget-container.S2,
#aftercontent.third .widget-container.S3 {width: 28.333%;}

#aftercontent.fourth .widget-container.S1,
#aftercontent.fourth .widget-container.S2,
#aftercontent.fourth .widget-container.S3 {width: 19.666%;}

@media only screen and (max-width: 959px) {

  	#aftercontent.half .widget-container.S1,
	#aftercontent.half .widget-container.S2,
	#aftercontent.half .widget-container.S3 {width: 45%;}

	#aftercontent.third .widget-container.S1,
	#aftercontent.third .widget-container.S2,
	#aftercontent.third .widget-container.S3 {width: 27.85%;}

	#aftercontent.fourth .widget-container.S1,
	#aftercontent.fourth .widget-container.S2,
	#aftercontent.fourth .widget-container.S3 {width: 19%;}
}

@media only screen and (max-width: 767px) {
	#aftercontent.half .widget-container,
	#aftercontent.third .widget-container,
	#aftercontent.fourth .widget-container { width: 100% !important;}
}