.wrap {
  display: flex;
  flex-wrap: wrap;
  flex-direction: column;         /* NEW */
  height: 100px;                  /* NEW */
  justify-content: space-between; /* NEW */
}
.elem1 {
  width: 20%;
  flex-basis: 100%;               /* NEW */
  border: 1px solid red;
}
.elem2,
.elem3 {
  width: 75%;
  flex-basis: 40%;                /* NEW */
  border: 1px solid red;
}