<div class="container">
<div class="block">Content Box</div>
<div class="block">Content Box</div>
<div class="block">Content Box</div>
</div>

/*
Calc Demo 100% Container, equally distribute containers within the box.
--
In this demo, we make the width of each box 100% divided by the number of boxes then subtract the margin left & right.
As you scale the page the box will always be 1/3 of the total width.
*/

.container  {
  width:80%;
  height:200px;
  margin:auto;
}

.block {
  width: calc(100% / 3 - 10px);
  height:100%;
  margin-left:5px;
  margin-left:5px;
  padding:10px;
  border: 10px dashed white;
}

/*
boring stuff
*/
html {
  box-sizing: border-box;
}

:root {
  background-color: #373150;
  font-size: 16px;
  font-weight: 400;
  text-rendering: geometricPrecision;
  line-height: 1.4;
  text-decoration-skip: ink;
  font-family: -apple-system-headline, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
}

*, *:before, *:after {
  box-sizing: inherit;
}

body {
  margin:0;
  padding:0;
}

.block {
  float:left;
  display:inline-block;
  background-color:#EF6174;
  color:white;
  font-size: 0.9rem;
  text-align: center;
  line-height:4em;
}