SVG: Multiple Effects in One Filter

You can use the result attributes to give a name to a filter primitive element's output, think of it as a sort of filter-local id attribute. You can then use that name as filter input with the in or in2 attributes.

<svg xmlns:xlink="http://www.w3.org/1999/xlink" width="1440" height="1750">
  <defs>
    <filter id="dropshadow">
     <feGaussianBlur in="SourceAlpha" stdDeviation="3" result="blur"/> 
      <feOffset dx="12" dy="12" result="offsetblur"/>
      <feOffset dx="-20" dy="-12" result="offsetblur2" in="blur"/>
      <feComponentTransfer result="shadow1" in="offsetblur">
        <feFuncA type="linear" slope="0.5"/>
      </feComponentTransfer>
      <feComponentTransfer result="shadow2" in="offsetblur2">
        <feFuncA type="linear" slope="0.2"/>
      </feComponentTransfer>
      <feMerge> 
        <feMergeNode in="shadow1"/>
        <feMergeNode in="shadow2"/>
        <feMergeNode in="SourceGraphic"/> 
      </feMerge>
    </filter>
  </defs>

  <path xmlns:xlink="http://www.w3.org/1999/xlink" d="M 100 100 L 300 100 L 200 300 z z" fill="#2DA9D6" filter="url(#dropshadow)"></path>
</svg>

See fiddle.