Yes, you need to apply for a China transit visa in Canada for your three days stop in Beijing.
The requirements are:
1. Passport with at least 6 months remaining validity and available blank pages for visa;
2. One properly completed visa application form.
3. A recent passport-size color photo with white background stuck on the visa application form.
4. a valid visa of the country of destination;
5. an air ticket showing transit through China.
the Chinese Embassy in Canada does not accept any straightforward application sent by mail or courier. Those who wish to apply for Chinese visa or document authentication by mail or courier need to send the applications to their clients or travel agencies in Ottawa, such as "Golden Mile 2000 Travel Consultant", who will submit the applications to the Embassy for processing on behalf of the applicants.