<section class="section top grad_bg overflow-hidden">
  <p class="text-center">上のコンテンツ</p>
</section>
<section class="section center">
        <div class="text-center grad_bg position-relative overflow-hidden">
            <div class="wrapper">
                <p class="text-center">真ん中のコンテンツ</p>
            </div>
        </div>
</section>

.section.top {
  padding: 4.4em 0 5.7em;
  position: relative;
  z-index: 1;
  min-height: 8em;
}
.section.top::before {
  display: block;
  content: "";
  position: absolute;
  left: 0;
  border-style: solid;
  border-width: 17vw 50vw 0;
  border-color: #fff transparent transparent;
  z-index: -1;
  bottom: 1.5em;
}
.section.top::after {
  display: block;
  content: "";
  z-index: -1;
  position: absolute;
  left: 0;
  background-color: #fff;
  height: 72%;
  height: calc(100% - 17vw);
  width: 100%;
  bottom: calc(1.5em + 17vw);
}
@media screen and (max-width: 680px) {
  .section.top {
    padding: 2em 0 1.5em;
  }
  .section.top::before {
    bottom: calc(23.5vw);
  }
}

.section.center {
  background-size: 100%;
  background-repeat: no-repeat;
  background-position: top;
  margin-top: -1.25%;
  padding-top: 1.25%;
  position: relative;
  color: #fff;
}
.section.center .grad_bg {
  padding-bottom: 6.5em;
  z-index: -1;
  min-height: 10em;
}
.section.center .grad_bg::after {
  display: block;
  position: absolute;
  content: "";
  border-style: solid;
  border-width: 17vw 50vw 17vw;
  z-index: -1;
  bottom: -17vw;
}
.section.center .grad_bg::after {
  border-color: transparent #fff #fff #fff;
}
.section.center .grad_bg::before {
  border-color: transparent transparent #fff transparent;
}
@media screen and (max-width: 680px) {
  .section.center {
    padding-top: 0em;
  }
  .section.center .grad_bg {
    padding-bottom: 6em;
  }
  .section.center .grad_bg::after {
    bottom: -10vw;
  }
}
.grad_bg {
  background-image: linear-gradient(90deg, #22af73, #00a397);
}