(2) The frequency ratio of each group is 3: 4: 9: 8: 6: 2 from left to right,
∴ If the frequency of 1 group is 3a, the frequencies of other groups are 4a, 9a, 8a, 6a, 2a,
The frequency of the third group is 9,
∴9a=9，
∴a= 1，
∴ The number of students selected in this survey is 3a+4a+9a+8a+6a+2a = 32a = 32;
(3) According to (2), the number of people with 90 points or above is 8+6+2= 16.
∴ The proportion of students with 90 points or above in mathematics is 1632× 100% = 50%.