File while the three-year window still contains a full year of foreign employment after US days are excluded. If a long US secondment is planned, file before it starts, because time in the United States after filing does not accumulate as foreign employment.
Count backwards from the filing date
Hypothetical example: the manager has been with the West Kelowna parent for three and a half years and the owner wants her to spend the coming spring in Washington getting the subsidiary's season started, then file in the summer. Counting backwards from a summer filing date, the three-year window would include the spring months in the United States, which do not count, plus the earlier Canadian employment, which does. There would still be well over a year of Canadian employment, so the timing works, but it is the kind of arithmetic that should be done on paper before the secondment is booked.
The riskier pattern is a manager who joined the parent only fourteen months ago and has already spent three months in Washington; the remaining eleven Canadian months fall short of the year, and the petition must wait until the count reaches twelve. In that scenario the spring secondment should be cancelled or shortened, because every additional US day delays eligibility.