How often should someone with osteopenia get a dexascan?

Every 1-2 years. Every 2 if mild and on no meds, once a year if there are other risks factors and on meds (if insurance covers it).
Varies. Bone changes very slowly, so if your first dexa shows very mildly low bone density, you might not see much significant change in a year, so 2-3 years may be reasonable. If you are borderline almost osteoporotic or have additional risk factors such as strog family history or steroid use, then annually. There is no consensus, and determined on case by case basis.
Depends. I would recommend starting reasonable calcium intake, some type of weight bearing exercise and recheck in 2years.