How do I install a subfloor over a concrete floor with a 3 inch dip toward the drain?

We are trying to finish half our basement. We have a concrete floor that slopes toward a drain that is not inside the half of the basement we are finishing. The height of the walls at the four corners and the three outside walls is 85", but the height of the inside wall (the wall through the middle of the basement) in the center is 88", which lead us to believe that our floor slopes quite a bit toward the drain. We found out we can't use Dricore with that much of a slope, and it would take over 20 bags and $600 of the self leveling concrete to level our floor. We ume that our only option is to build a subfloor over the concrete slab, but we are unsure how to do so with such a slope. Any thoughts?