@RuthHeath Welcome to the Canvas Community! There is not a way in the Rich Content Editor to add tabs, it all has to be done through the HTML view using the < /> button in the bottom right. I don't believe enhanceable elements are officially supported by Canvas anymore but the tabs one still works. Here is the code:
<div class="enhanceable_content tabs">
<ul>
<li><a href="#fragment-1">PUT TITLE FOR TAB HERE</a></li>
<li><a href="#fragment-2">PUT TITLE FOR TAB HERE</a></li>
<li><a href="#fragment-3">PUT TITLE FOR TAB HERE</a></li>
</ul>
<div id="fragment-1">PUT THE CONTENT FOR THE FIRST TAB HERE</div>
<div id="fragment-2">PUT THE CONTENT FOR THE SECOND TAB HERE.</div>
<div id="fragment-3">PUT THE CONTENT FOR THE THIRD TAB HERE</div>
</div>
Once you paste the above code in, you can switch back to the regular editor and add content, just be careful.
Hope this helps!
-Nick