[c#] Limiting double to 3 decimal places

If your purpose in truncating the digits is for display reasons, then you just just use an appropriate formatting when you convert the double to a string.

Methods like String.Format() and Console.WriteLine() (and others) allow you to limit the number of digits of precision a value is formatted with.

Attempting to "truncate" floating point numbers is ill advised - floating point numbers don't have a precise decimal representation in many cases. Applying an approach like scaling the number up, truncating it, and then scaling it down could easily change the value to something quite different from what you'd expected for the "truncated" value.

If you need precise decimal representations of a number you should be using decimal rather than double or float.