Model looks pretty good, especially for a first. I think the HDRI renders are blotchy because of your final gather settings, you may need to boost the accuracy a wee bit and also increase the point interpolation and go from there. Personally I don't think HDRIs are a cure all, as it can be a pain in the neck at times, its nice to have a backdrop being reflected but its even better to have the reflections make sense or even be abstract, its kinda becoming a cheesy effect as many people are using the same 3 HDRIs circling the net. A few light cards, maybe even some textured geo would help in the reflections department. As far the render with the three point lighting, did you make the shadow color lighter? I ask because its really disappearing onto backdrop and making the camera look like its floating. Also it seems to be missing a rim light.