Key word here is ‘recommended’ should enter which simply of course is saying a good idea to enter at least 3 weeks before simply as a recommendation , assume to get accommodation and so on sorted. If it was a mandate would say should not enter more than 3 weeks before start date.It is not being told by someone
When your visa get approved
U got an confirmation mail. For jan 2019 intake additional line has been written on the mail that "it is recommented that u should enter canada 3 weeks before your course starting date"
No idea why as implied some people have this and others do not other than maybe down to the visa officer issuing or based on applicants country.