A basic sloped 3D floor
Using action specials 160 and 181
A tutorial for GZDoom
1. build the game sector

2. build the sectors which are to be the 3D floor
and give those sectors an ID tag, for example 11 and 12

3. build the control sectors
note theceiling and floor heights for the floor thicknesses
make sure that the control sectors align with the game sectors

4. attach to one of each linedef of the control sector the action special 160
and copy the propriate sector tag, 11 and 12 respectively

5. add the control sectors for the sloping
again, note ceiling and floor heights

6. attach the action special 181 to the linedefs
and set the way it needs to be sloped, front or back

7. preview in the editor
apply textures to have the 3D floor to be more visible

8. what it looks like in-game

One could ask if it is possible to have another set of 3D floors on top of this one. The answers
would be: of course it is possible and it is easy to accomplish.
Copy the control sectors and in Height offset type in the increase of the new height for
the added 3D floors, maybe ++64.
And as before, make sure that the sontrol sectors align with the game sectors.
what this looks like in-game