JSFiddle - React, Tailwind, and code Playground

HTML

<div id="main">
   <div class="affianca">
      box3
   </div>
   <div class="affianca">
      box2
   </div>
</div>

CSS

#main{
  position: absolute;
  width: 200px !important;
  height: 100px !important;
  top: 50%;
  margin-top: -50px; /* la metà dell' altezza */
  left: 50%;
  margin-left: -100px; /+ la metà della larghezza */
}

.affianca{
  float:left;
}