From the following, I can't tell whether the three months would run from the first or second chest xray. Why not call the DMP you are dealing with and ask them?http://www.cic.gc.ca/english/resources/publications/dmp-handbook/appendix-02.asp
Question 7: Tuberculosis, sexually transmitted diseases, etc.
When there is a past history of tuberculosis, the details from the applicant should include the date of diagnosis, duration and type of treatment. In all applicants, whether excessive demand exempt or not, copies of previous treatment reports, x-rays and other relevant information should be obtained and forwarded with the immigration medical forms, if possible. If the medical history reveals any suggestion of previous tuberculosis, then clinical, bacteriological and radiological examinations will be required to determine the activity of the disease.
Where there is an abnormal chest radiograph but no history of previous active tuberculosis, or there is a history of previous tuberculosis with or without previous adequate treatment, the DMP should generally note the abnormality and send the file in for review by the regional medical office. In most cases the file will be furthered until:
•a minimum of two chest films, taken at a minimum interval of three months, have a stable appearance; and
•three sputum samples taken at least 24 hours apart, examined for acid-fast bacteria (smear), and incubated for six to eight weeks for tubercle bacilli by standard culture methods are negative.
In the absence of secretion bacteriology and culture, serial chest radiographs showing stability of abnormalities compatible with pulmonary tuberculosis for greater than six months may be requested.
All applicants who are diagnosed clinically, radiologically or bacteriologically as having active pulmonary tuberculosis must be treated and rendered “inactive” before entry to Canada.