Why three SSNs can be a sensible first batch

Would buying only three SSNs be an acceptable first Canadian nuclear-submarine order?

Yes; three boats are enough to begin building national skills before later batches expand the force.

A first purchase of three nuclear attack submarines is considered as the beginning of a Canadian programme. British and American examples illustrate starting with one or two boats before buying more. The initial order provides a way to develop skills and begin generating a force. That reasoning concerns an opening investment; it does not establish that three boats would satisfy Canada's eventual operational requirements.