You could try using the align & distribute palette. Make a comp that is the width of 1000 images + 999 X 3 pixels wide, then place one image at the start, one at the end, select them all, then hit the button that distributes the images by left edge. That should work.
If you’re keen on expressions, try using this layer index-based expression for each layer’s position:
[width/2*(2*index-1) + (index-1)*3, position[1]]
or:
xPos=width/2*(2*index-1);
AddGap=(index-1)*3;
[xPos + AddGap, position[1]]
Apply it to the first layer, select the property (position), then edit>copy expression only. Then select the other layers and select edit>paste.
For flexibility, I might tie the 3 value to a slider.
Dan will probably come up with something cleaner. 🙂
Steve