Answers (1)
Everybody in USA uses the period to separate units digit from tenths digit. I have to assume that you mean 12.09949 since you are asking about decimals. You should be aware of the varying customs.
Three decimals means the third digit to the right of the point: 12.099_ The next digit is less than 5 so you drop everything after that. If it had been 5 or higher, you would drop it and add 1 to the third digit.