Through interview.
Without hard and fast rules as published, the selection job is just impossible. Interview is the only mechanism which provides a very limited room for manoeuvre for the DSE-route students.
Based on the hypothetical situation, the 3 students differ only in subjects taken and the scores. In case the total scores are very marginal, I would say student 1 may not even be granted an interview.Though Biology is preferred, it is absolutely not mandatory and I do not see it as important to make up the 1 mark shortfall. There are quite many medic students not having taken Biology. So I think student 1 stands a less chance. However, I would suggest you let your kid study what he/she is interested than just for making the grades.