Adding More Than One Category to Category Results

On my website,, I have more category names than I want displayed across the green navigation bar at the top. However, I would like multiple categories to be showed in the category results page when it is clicked on in the navigation bar. For example, right now, when you click on the word "Health" it displays the items from the category body-remedies only. I would like it to display all items from all of the following 3 categories instead: /health-remedies/brainy-treats and /health-remedies/body-remedies and /beauty-remedies/mouth-remedies

How can I change this? If it helps, I've granted support access through the wpmu plugin.

Thanks for your help!