Through a port of entry, yes, provided the letter and degree documents are ready. A USCIS petition would be unlikely to be adjudicated in that window without premium processing.
Three weeks is enough only if the letter arrives in week one
Hypothetical example: a Frontenac Islands instructor accepts an appointment in mid-August for a September start. The port-of-entry route makes the date achievable because there is nothing to wait for except the documents. The realistic constraint is the college, which must produce a letter that names the profession, the degree requirement, the duties, the salary and the duration; a public institution's signing process can take longer than the applicant expects.
She should send the college a specification of the letter on the day she accepts, obtain an official transcript from her university at once and book the crossing for a weekday that leaves room for a second attempt if the officer asks for something additional. She should also confirm whether the college's own onboarding, such as a background check, has its own lead time that could delay the actual first day regardless of status. Hypothetical example: A Frontenac Islands history instructor is expected to begin orientation on a fixed August date.
Her first review should work backward from that date: signed letter, official transcript, any required licence, and time to correct a missing item. The dean’s signed appointment letter settles whether the job is final rather than tentative. She should not resign a Canadian position until the admission decision and the employer’s onboarding conditions are clear.