New menu appears but also replaces primary nav.

In my theme I have two menus that appear on every page (primary nav in header, social media nav in footer). I am trying to add a third menu (a “filter posts by category” menu) in the body of the blog page only.
I built all three in the menus section of the dashboard. I registered all three in functions.php. I have designated each to it’s own spot in the “manage menus” tab. I added the php to the singular page where the “filter” menu is to appear.
When I do this, the menu appears where it should be, but also replaces the menu in the header on every page. The footer menu (social media) remains untouched.
When I tried to go back a step and get the primary nav back, I had to delete the menu from the Menu section (commenting out the code in the blog page and functions did not work).
Any idea where I went wrong? Did I miss a step?
Thanks for any ideas on how to fix this!