Generated by SassMeister.com.

SassMeister-input-HTML.html

<div class="container">
    <div class="gallery">
      <div class="gallery__item">
        <img src="http://placehold.it/480x270" alt="">
      </div>
      <div class="gallery__item">
        <img src="http://placehold.it/480x270" alt="">
      </div>
      <div class="gallery__item gallery__item--third">
      <div class="gallery__item--height">
          <a href="#">
            <img src="http://placehold.it/480x270" alt="">
            <span>Some Text</span>
          </a>

          <a href="#">
            <img src="http://placehold.it/480x270" alt="">
            <span>Some Text</span>
          </a>
        </div>
      </div>
    </div>
  </div>

SassMeister-input.scss

// ----
// Sass (v3.3.14)
// Compass (v1.0.3)
// Susy (v2.2.5)
// ----

@import "susy";
$susy: (columns: 12, gutters: 0.25, container: 1140px, debug: (image: show-columns));

img {
  max-width: 100%;
  height: auto;
}

.container {
  @include container;
}

.gallery__item {
  @include gallery(4 of 12);
}

.gallery__item--third {
  .gallery__item--height {
    position: relative;
    height: 0;
    padding-bottom: 56.25%;
  }

  a {
    position: absolute;
    top: 0;
    left: 0;
    right: 0;
    display: -webkit-flex;
    display: -ms-flexbox;
    display: flex;
    -webkit-align-items: center;
        -ms-flex-align: center;
            align-items: center;
    height: calc((100% - #{gutter(4)})/2);
  }

  a:last-child {
    top: auto;
    bottom: 0;
  }

  img {
    height: 100%;
    width: auto;
  }
}

SassMeister-output.css

img {
  max-width: 100%;
  height: auto;
}

.container {
  max-width: 1140px;
  margin-left: auto;
  margin-right: auto;
  background-image: linear-gradient(to right, rgba(102, 102, 255, 0.25), rgba(179, 179, 255, 0.25) 80%, transparent 80%);
  background-size: 8.47458%;
  background-origin: content-box;
  background-clip: content-box;
  background-position: left top;
}
.container:after {
  content: " ";
  display: block;
  clear: both;
}

.gallery__item {
  width: 32.20339%;
  float: left;
}
.gallery__item:nth-child(3n + 1) {
  margin-left: 0;
  margin-right: -100%;
  clear: both;
  margin-left: 0;
}
.gallery__item:nth-child(3n + 2) {
  margin-left: 33.89831%;
  margin-right: -100%;
  clear: none;
}
.gallery__item:nth-child(3n + 3) {
  margin-left: 67.79661%;
  margin-right: -100%;
  clear: none;
}

.gallery__item--third .gallery__item--height {
  position: relative;
  height: 0;
  padding-bottom: 56.25%;
}
.gallery__item--third a {
  position: absolute;
  top: 0;
  left: 0;
  right: 0;
  display: -webkit-flex;
  display: -ms-flexbox;
  display: flex;
  -webkit-align-items: center;
  -ms-flex-align: center;
  align-items: center;
  height: calc((100% - 5.26316%)/2);
}
.gallery__item--third a:last-child {
  top: auto;
  bottom: 0;
}
.gallery__item--third img {
  height: 100%;
  width: auto;
}

SassMeister-rendered.html

<div class="container">
    <div class="gallery">
      <div class="gallery__item">
        <img src="http://placehold.it/480x270" alt="">
      </div>
      <div class="gallery__item">
        <img src="http://placehold.it/480x270" alt="">
      </div>
      <div class="gallery__item gallery__item--third">
      <div class="gallery__item--height">
          <a href="#">
            <img src="http://placehold.it/480x270" alt="">
            <span>Some Text</span>
          </a>

          <a href="#">
            <img src="http://placehold.it/480x270" alt="">
            <span>Some Text</span>
          </a>
        </div>
      </div>
    </div>
  </div>