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Bootstrap class: .flex-fill

<div class="d-flex text-white">
  <div class="p-2 flex-fill bg-primary">Flex item</div>
  <div class="p-2 flex-fill bg-success">Flex item</div>
  <div class="p-2 flex-fill bg-primary">Flex item</div>
</div>

Preview

Flex item
Flex item
Flex item