<div class="Wrapper">
  <div>
    <h4>Button with "display: flex" on it</h4>
    <button class="Button Button--flex">
      <div class="Button-element">1</div>
      <div class="Button-element">2</div>
      <div class="Button-element">3</div>
    </button>
  </div>

  <div>
    <h4>Button with inner div with "display: flex" on it</h4>
    <button class="Button">
      <div class="Button-elementWrapper">
        <div class="Button-element">1</div>
        <div class="Button-element">2</div>
        <div class="Button-element">3</div>
      </div>
    </button>
  </div>
</div>

.Wrapper {
  display: flex;
  justify-content: center;
  flex-wrap: wrap;
}
  
 .Wrapper > div {
  margin: 20px 10px;
}

h4 {
  font-family: Helvetica, Arial, sans-serif;
  margin-bottom: 15px;
}

.Button {
  width: 300px;
  height: 100px;
  padding: 0;
  background: #f6f7f9;
  border: 1px solid #ddd;
}

.Button--flex {
  display: flex;
  justify-content: space-around;
  align-items: center;
}

.Button-element {
  flex-basis: 80px;
  border: 1px solid #789;
  line-height: 80px;
  height: 80px;
}

.Button-elementWrapper {
  display: flex;
  justify-content: space-around;
  width: 100%;
  align-items: center;
}