There is a fairly significant improvement in the Cd by having a single volume chassis; Mercedes studied this and that is what they concluded. The "interference" and interaction between the three volumes in your model will raise the drag more than the reduction of the frontal area.
Keep in mind that the wheels are spinning - the top of the wheels are moving forward at twice the speed of the vehicle.
The underside can't be sloped as much as the side and top; since the ground constricts the air flow, and fully covering the rear wheel will help lower the drag significantly.
Once the surfaces are tapered, they need to stay at the same angle or curve. The air needs to smoothly close back together, and the straightening on the tail of your model will disrupt this, and cause more turbulence than if you removed the "concavity" on the sides; or better yet narrowed the back.
In an ideal shape, the top and the sides can be tapered more than the bottom, and that if the vehicle is roughly square in section, this means that the sides will determine the length; forming a "fish tail" rather than a "beaver tail".
With a tadpole trike, you need the two wheels to be pretty far apart for stability, so the section will be roughly rectangular, so the top and the bottom tapers might allow you to keep the beaver tail. You also could consider two rear wheels with a narrow track, so you can keep the width in the front to a manageable dimension. Take a long hard look at Dave Cloud's Dolphin for a nearly ideal aerodynamic shape:
The *Ultimate* Aerodynamic Car? Dave Cloud's "Dolphin"
I've made a model (in SketchUp) of the Dolphin:
SketchUp Models of Concept Vehicles