I have had a 7 CM ovarian cyst for 3 years, is this common?

Cysts are. But a 7cm ovarain cyst is relatively large and m,ay cause some pain, etc. Have the cyst followed regularly by the OB/GYN Dr/surgeon and it may need to come out, especially if it causes you pain and or has the possibility to become "torsed" where it would then require emergency surgery. Best wishes.
Yes. Small ovarian cysts are common when not on hormonal contraceptive suppression and generally are ignored if there are no symptoms by the patient. However, your age of 53 warrants at least a laparoscopic evaluation and sampling of the cyst to be certain there is no malignancy since it has persisted x 3 years.
Yes. Not all ovarian cysts are cancerous. You cyst is quite large & I assume that your gynecologist is following it & has made a judgement to just keep an eye on it if it has not changed.