How to find element that has multiple class in selenium

I have an element with 3 classes which I need to find with selenium

 <button style="padding:2px 7px; background-color:#4caeea" 
  class="btn btn-xs btn-custom" </button>

I could not find it with : By.classname("btn btn-xs btn-custom")

I do not want to use xpath & cssSelector . What other options do I have ?