A Canadian citizen can apply at the port of entry on the way to work, but the experience evidence and employer letter should be finished weeks earlier. An I-129 petition adds USCIS processing time before entry.
Build the evidence timeline backward
Hypothetical example: a Quispamsis applicant is asked to start on the first of the month, five weeks away. A port-of-entry application can be made on the day of travel, so the constraint is not the application but the preparation: employment-verification letters from two former employers, a transcript request from the college and a properly drafted employer letter can each take two or three weeks. If the employer prefers to file Form I-129, USCIS processing must be added before travel, and the start date should be negotiated accordingly.
The applicant should also allow time for a possible refusal and re-application if the officer wants more evidence about the three-year alternative. Setting the resignation date after the letters are in hand, rather than the day the offer arrives, keeps the timetable honest.