For a non-emergency MRI three days is not too long. They will be read by a radiiologist that night or the next day. (S)He will dictate a report, which will be transcribed the following day. The radiologist will usually look through the reports and approve/sign them the day after. They will then be sent to the ordering physician. If faxed, that will be the third or fourth day; if sent by snail mail, it will depend on thecity of origin and the city of the destination. That said, there would be nothing too amiss in calling and asking how it's coming, and when your doctor might receive a report.
I am a physician specializing in occupational and environmental medicine. I am not an authority on HMS or EDS, but find I have several patients with the condition and am trying to learn more.