You would need to set the count to 3. Then, you need to add another Recent Post shortcode but this time, you need to add the offset attribute. The code should look like this:
[recent_posts type="portfolio" count="3"]
[recent_posts type="portfolio" count="3" offset="3"]
In case you don't like that setup, we have Essential Grid and The Grid bundled in X / Pro so you have more options to choose from.
Hope that helps.