3. When must I submit the supporting documentation?

This depends upon the category of Canadian immigration under which you are applying and the Canadian Immigration Visa Office to which you are submitting your application.
Under the Skilled Worker category of Canadian immigration, all documents need to be submitted along with your e-application through the Express Entry system. Language test results and education credentials need to be submitted when you create your online profile.
Under the Family Sponsorship category of Canadian immigration, all supporting documents including Police Clearance Certificates and proof that a medical examination has been undergone by the sponsored person(s) must be submitted with the initial application.
Under the Entrepreneur and Self-Employed Persons business categories of Canadian immigration, as a general rule you submit only a basic application form and Canadian government processing fees to the Canadian Visa Office responsible for your country of citizenship or for the country in which you are currently residing if you were lawfully admitted to that country for at least one year. Approximately four months prior to the assessment of your application you will be requested to submit supporting documents in relation to your civil status and business management experience.
Under the Investor business category of Canadian immigration, all supporting documents must be submitted to the appropriate Canadian Immigration Visa Office at the time that the application is initially submitted.
Under the Provincial Nominee Program of Canadian immigration or if you are intending to reside in the Province of Quebec, all supporting documents in relation to the provincial nomination must be submitted with the initial application to the province. Under the Canadian Experience Class category of Canadian immigration, all supporting documents must be submitted along with your e-application through the Express Entry system. Language test results are required when you create your online profile.