This is interesting. From an immigration point of view, what the officer wants to find out is simple - is the applicant actively enrolled as a student throughout the 3 years.
Please note, enrollment simply means officially paying for the semester even while you are on work term.
Here is another analysis: During your work term, did you get assessment criteria from your school or the organization you were working with? I.e. do they have a structure for assessing your performance during the work term. Grading, attendance, performance, site visits etc. Did you take your performance sheet back to your school before they awarded you the credits?
Because, if you were actively enrolled as a (paying) student during your work term and there is a structure in place to measure or grade your performance during this period, then it counts towards the programme. Otherwise, your work term is just a process or pre-requisite for the programme and it wouldnt count as part of academic period.
Things can get really complex when an immigration officer assess candidate eligibility for a visa. In my personal opinion, I think it is safer to choose 15 points instead of 30points.