"successful sinus surgery 3 years ago. Now opening to sinuses partially blocked scar tisssue. How common is this, how involved is surgery"?

It happens. A raw area can scar to another portion of the nasal tissue and block off the sinuses. This does happen sometimes and there are steps that can be taken to reduce this in the surgery. However once you have it you should get it corrected. The second surgery should be relatively simple and sometimes can be done in the office with anesthetic. If it is extensive, then you should go to the or.