<div class="flex-container box">
  <div class="box__item"></div>
  <div class="box__item search"></div>
  <a class="box__link" href="#">ссылка</a>
</div>

.flex-container{
  display:flex;
  border:1px #cc0000 solid;
  padding:10px;
  flex-wrap:wrap;
}
.box__item{
  background:#ccc;
  flex:1;
  margin:3px;
  min-height:200px;
}
.box__link{
  flex:none;
  display:inline-block;
  margin-right: 100%;
}

.search{
      width: 100%;
    max-width: 160px;
}