extruding 3 faces to get a wall i´m totally new to blender, so i want to learn the basics as good as possible.

My problem: I want to make just a simple Studio background with 1 floor and 2 walls by extruding 3 faces of a cube. (cube, delete 1 vertex, select 3 faces and extrude).
When i do so, the new walls are slanted or their origin is just 45° below the old origin.
I found a workaround: setting the 3d cursor to the first deleted vertex and then extruding the walls by 0, then scaling the new walls with my 3d cursor as center. But i want to do it just by extruding.
what am i doing/thinking wrong?
If you look in the upper left corner when you are doing the Extrude Along Normals you will see that you can change the behaviour. In this case you can press S or Alt to get Even Thickness.



Even Thickness is also an option in the Solidify Modifier.