不幸的是，在当前版本的CSS网格中似乎没有像 grid-column这样的真正魔术：span auto / -1 作为通用解决方案。但是对于这种3x1网格的特殊情况，您可以执行以下操作：

Unfortunately, it seems that in current version of CSS grid there is no true magic like grid-column: span auto / -1 as a universal solution. But for this particular case of 3x1 grid you can do something like the following:

.row {
  display: grid;
  grid-template-columns: repeat(3, 1fr);
}

.col {
  background: blue;
  padding: 20px;
  border: 1px solid red;
}

.col:last-child {
  background: yellow;
  grid-column-end: -1;
}
.col:nth-child(1):last-child {
  grid-column-start: 1;
}
.col:nth-child(2):last-child {
  grid-column-start: 2;
}

<div class="row">
  <div class="col"></div>
  <div class="col"></div>
  <div class="col"></div>
</div>
<div class="row">
  <div class="col"></div>
  <div class="col"></div>
</div>
<div class="row">
  <div class="col"></div>
</div>