try this in the plain text editor:
<div class="grid-container">
<div class="grid-33">
<h1>I'm a 33,3333% Column</h1>
<p>And here you have a lot of content, you know, just to fill up all this space there's a grid that is already integrated in generatepress and is not that difficult to use it.</p>
</div>
<div class="grid-33">
<h1>I'm a 33,3333% Column</h1>
<p>And here you have a lot of content, you know, just to fill up all this space there's a grid that is already integrated in generatepress and is not that difficult to use it.</p>
</div>
<div class="grid-33">
<h1>I'm a 33,3333% Column</h1>
<p>And here you have a lot of content, you know, just to fill up all this space there's a grid that is already integrated in generatepress and is not that difficult to use it.</p>
</div>
</div>
And let me know if it works for you.