To round numbers to the nearest thousandths, you have to pretend that you're using a number grid, starting with 0, where numbers are 0.001 units apart, and there's nothing in between.
So, the grid looks like

Now the two numbers in this grid which surround 0.9836 are 0.983 and 0.984, and we have to choose the closest one. Since

we can see that 0.984 is less distant from 0.9836, and so we can round 0.9836 to 0.984