Navigation bar on small screen

I’m sure this is covered in the documentation somewhere, but I don’t know where to look. I have a navigation bar in my header with three items. I finally got it to look the way I want on a desktop and discovered that when the website is opened on a phone the navigation bar is converted into a dropdown menu, but the type for the three items is such that they are all on top of each other so it is not possible to select one of them. How do I control the appearance of this dropdown menu separately from the appearance of the navigation bar?

The page I need help with: [log in to see the link]