I decide where to put my digits by looking at the numbers I have and trying to see if they make 800 or 900 because then the tens column and one's column will add up to get you closer to 1000. If not, then put your small numbers in the hundreds column and then put your bigger numbers in the ten's columns because if your big numbers are in the hundreds, they won’t be near 800, 900 and 1000.
My strategy is to make around the 850 mark with my hundreds and then pit the tens and ones in a place. I will then add the three 3-digit numbers and move them around depending on what the three-digit numbers made. My strategy would work the best of I hit the 800 - 900 mark with large tens and ones, but it would not work if I had massive numbers in each column.
The closest I got was 1012 by having large tens and ones and small hundreds, and i moved the numbers around to get to the target a little bit closer.