For part (iii), may i know if the answer of 72/105 is from your teacher? Considering that 72/105 is more than 1/2, the probability seems too high, so is quite doubtful to be correct.

I am interpreting the question as saying that the group of 3 students consisting of chairperson, vice-chairperson & secretary, are standing in the center of the row, occupying the 3rd, 4th & 5th positions (in various order).

Another possible interpretation may be that either one of the 3 (chairperson, vice-chairperson or secretary) is standing at the exact center (4th position), but I feel that this is an incorrect interpretation.