In order to make the 6-digit 15 □ □ 6 divisible by 36 and get the largest quotient, it should be filled in______ ．

In order to maximize the quotient, the divisor should also be the largest, so the thousand digits can be filled with 9; and the divisor is a multiple of 4, so the ten digits should be filled with 1, 3, 5, 7, 9. At this time, the corresponding hundreds should be filled with 5, 3, 1, 8, 6. So the number in the three cabinets is 987. So the answer is: 987