So although there's an "anchor" behavior, I find it to be pretty bad and jittery. What I do is create a "GUI" layout, rename the layer to "GUI", mark it as global, and set the "Parallax" under "Screen & zoom" for the layer to 0% x 0%. Note that this layer will override any other layout's layer named "GUI".
Now all you have to do is add a layer called "GUI" to your other layouts and boom, fixed position GUI!
Of course you could call it like "HUD" or "Controls", and just add that layer to any layout you want the HUD/Controls to be visible in. The import thing is that you add a layer with the same name to any layout you want the controls visible in as C3 will replace it with the global layer.
And in case you are wondering, Parallax is supposed to allow the layer to scroll at a different rate than the rest of the layout. By setting it to 0% x 0%, you're forcing it to never scroll (giving you a stable fixed position on the screen at all times).