The next larger thousandth is 36.994 .
The next smaller thousandth is 36.992 .
Neither of those is any nearer to 36.993
than 36.993 already is.
The last '3' at the end of 36.993 is in the thousandths' place.
There is no more piece of another thousandth after it.
So 36.993 is already on a complete thousandth, and
there's no rounding required.