You want to create a custom roof slab with 3 different corner heights.
How to create roof slab from lines.
Click duplicate to create a new type.
To add a roof slab you simply specify two points and an angle.
Explode x the 3d polyline into separate lines.
Select element properties type properties.
Use the thicken command to convert the surface into a 3d solid with the desired roof thickness.
Revit will open contextual tab in your ribbon.
Open the tool palette that contains the slab or roof slab tool you want to use.
Select a closed polyline and press enter.
Draw a 3d polyline 3dpoly along the edge of the walls to outline the shape.
You may have to scroll to display the desired tool right click the tool and click apply tool properties to linework and walls or linework walls and roof in the case of roof slab tools.