Sketchfab model embeds are supported in Tumblr posts.
Create a new Text post. Copy/paste your embed code into Tumblr's HTML editor mode.
Keep in mind that switching back to Markdown or Rich Text may break the iframe code, so you should stay in HTML mode or finish the rest of your post in another mode first.
We've also experienced issues with iframes when re-editing a post, especially if you're using a lot of custom styles and code.