Ok, so maybe this autotext is what you need. Add a text on a master layout (grey icon, I marked it on the screenshot) in a layout book, in the text editor in the top left corner you have the option to Insert Autotext. From the list that will appear you can choose 'Drawing Scale'. It will show as 1:### on a master layout, but as soon as you place a view on a layout, Archicad will read the scale of your drawing and show it automatically. The best in your case is to make two master layouts (A1 and A3), place autotext for drawing scale on both of them. Create two different layouts using these 2 master layouts, and you can place the same view (for example floor plan) on both layouts, in different scales (i.e.1:50, 1:00).
Layout will show the scale that was set for the drawing (or saved within the view):
I hope this is what you were looking for!