To make three numbers which add up to the largest sum possible, we want to maximize each number. In order to do that, we want to push the larger numbers as far to the left of the decimal point as possible. For instance, if we place 9 in the units column, it's just 9. If we place it in the hundreds column, it's 900. This gives us more bang for the buck. It doesn't matter which three digit number contains which particular digits, since they'll all be added together. We just want to make sure the largest numbers are as far to the left as possible.
The largest numbers are 9, 8, and 8. Let's put those in the hundreds column:
The next largest numbers are 4, 5, and 6. Let's put those in the tens column:
The remaining numbers are 0, 1, and 1. Let's put those in the remaining spots:
940 + 851 + 861 = 2652