<div class="box">
 <div class="box__content">
    бла бла бла
  </div>
</div>

body {
  background: #6733e3;
}

.box {
  position: relative;
  margin: 20px auto;
  max-width: 400px;

  &__content {
    position: relative;
    padding: 40px;
    z-index: 100;
    background: #fff;
    text-align: center;
    border-radius: 10px;
  }
  &:before,
  &:after {
    position: absolute;
    top: 0;
    left: 0;
    bottom: 0;
    right: 0;
    border-radius: 10px;
    content: "";
  }
  &:before {
    z-index: 90;
    margin-bottom: -20px;
    margin-left: 10px;
    margin-right: 10px;
    background: #875ce8;
  }
  &:after {
    z-index: 80;
    margin-bottom: -40px;
    background: #7748e5;
    margin-left: 20px;
    margin-right: 20px;
  }
}