When you add some elements, this auto resize the boxes dividing 100% (width) by amount of this.

Auto-Size float boxes

HTML Before
<div class="boxes">
	<div></div>
	<div></div>
	<div></div>
</div>

$(document).ready(function(){

	var multiBox = $('.boxes div').size();

	if ( multiBox > 0 && $(window).width() > 768 ) { // only for desktop
		$('.boxes div').css('width', (100 / multiBox) + '%');
	} else {
		$('.boxes div').css('width', '100%');
	}

});


HTML After > 768px
<div class="boxes">
	<div style="width: 33,333333333%"></div>
	<div style="width: 33,333333333%"></div>
	<div style="width: 33,333333333%"></div>
</div>

HTML After < 768px
<div class="boxes">
	<div style="width: 100%"></div>
	<div style="width: 100%"></div>
	<div style="width: 100%"></div>
</div>