Forget the osteopenia. You have osteoporosis, and a pretty low score for someone so young.
It's unfortunate that there are a lot of gastric side effects with the bisphosphanates. That's why the drug companies have developed some meds that are given IV. A good one to consider is Reclast. It's given as a once a year IV and has proven results.
Your question about improving bone density, or even remaining stable, by electing not to go on prescription meds, is one that is often discussed here. The short answer is no, it can't be done. After you reach a certain point, the body will not rebuild bone naturally. Just won't work.
The risk you take with a -3.2 number is that you are at a higher risk for fracture and that bone will be harder to strengthen the longer you attempt the natural route. The lower the number and the times you fracture, increase your risk of having a life altering fracture.
If you asked any of our patients who've have a compression fracture if they would risk having another one or take a medicine shown to reduce that risk, every one would opt for the med. A compression fracture not only curves your spine and looks unflattering, it compresses your internal organs which can affect digestion, breathing, movement just to name a few, besides being very painful.