We want to know the sampling method used when a interview is done at every 3rd student that board nuses at the end of the school day.
As there is a fixed and periodic interval of time where the students are being interviewed, the sampling method won't be random.
As they don't ask the students if they want to participate or not on the interview, the sampling is not self-selected.
And also, as the starting point is random, but there is a systematic method determining how they interview, the sampling method is a systematic sample.