Where exactly do you want the sloped 3d floor to be? I'm checking the map out in Doom Builder but I'm not sure where you want the floor and which way you want it to slope.
I need the 3D floor to slope to that sector across it, but it does that, but I tried doing some stuff that I thought might work and now the 3D floor won't show up!
Make the control sector exactly like you want the slope to appear. This means make it the same length as the area you want the slope to be in. Also, align the control sector parallel to the sector where the slope is going to be. You also had the line with the slope special facing the wrong way, so it was sloping the wrong part. And last, put the line with the 3D floor special on the sloped part of the control sector. Now I'm not the greatest at explaining things, so here's an image to help you out (don't mind the crappy handwriting and image quality, I did it in paint real fast):