ערימות

<div class="vstack gap-3">
  <div class="bg-light border">First item</div>
  <div class="bg-light border">Second item</div>
  <div class="bg-light border">Third item</div>
</div>

<div class="hstack gap-3">
  <div class="bg-light border">First item</div>
  <div class="bg-light border">Second item</div>
  <div class="bg-light border">Third item</div>
</div>

<div class="hstack gap-3">
  <div class="bg-light border">First item</div>
  <div class="bg-light border ms-auto">Second item</div>
  <div class="bg-light border">Third item</div>
</div>

<div class="hstack gap-3">
  <div class="bg-light border">First item</div>
  <div class="bg-light border ms-auto">Second item</div>
  <div class="vr"></div>
  <div class="bg-light border">Third item</div>
</div>

<div class="vstack gap-2 col-md-5 mx-auto">
  <button type="button" class="btn btn-secondary">Save changes</button>
  <button type="button" class="btn btn-outline-secondary">Cancel</button>
</div>

<div class="hstack gap-3">
  <input class="form-control me-auto" type="text" placeholder="Add your item here..." aria-label="Add your item here...">
  <button type="button" class="btn btn-secondary">Submit</button>
  <div class="vr"></div>
  <button type="button" class="btn btn-outline-danger">Reset</button>
</div>

.hstack {
  display: flex;
  flex-direction: row;
  align-items: center;
  align-self: stretch;
}

.vstack {
  display: flex;
  flex: 1 1 auto;
  flex-direction: column;
  align-self: stretch;
}