Limiting double to 3 decimal places

This i what I am trying to achieve:

If a double has more than 3 decimal places, I want to truncate any decimal places beyond the third. (do not round.)

Eg.: 12.878999 -> 12.878

If a double has less than 3 decimals, leave unchanged

Eg.:   125   -> 125
       89.24 -> 89.24

I came across this command:

double example = 12.34567;
double output = Math.Round(example, 3);

But I do not want to round. According to the command posted above, 12.34567 -> 12.346

I want to truncate the value so that it becomes: 12.345