It took me a while too to figure it out
That said, the *best document* is not the Invitation letter or the Purpose of Visit... as neither of these documents have a logical connection to the previous refusals per se.
You must prepare a separate explanation + reference both the Refusal explanation and the refusal evidence (OP read: Refusal letters issued by the countries you had previously applied to) in Question 2 of the Background information section of the TRV application form.
You would also want to prove you have addressed the refusal reasons (if you were refused by Canada) in your cover letter