Rounding Numbers not consistent??? (C#)

So I made a timer that counts down 10 seconds and has 3 decimal places. The time that is getting taken away is making it have more then 3 decimals and I tried using “Mathf” but it would only work some times. I’ll add my code and a video on whats the timer doing.

using System.Collections;
using System.Collections.Generic;
using UnityEngine.UI; 
using UnityEngine;

public class timer : MonoBehaviour {

	private Text textBox; 
	public float timeTaken;

	// Use this for initialization
	void Start () {

		textBox = GetComponent<Text>();

	// Update is called once per frame
	void Update () {

		timeTaken -= Time.deltaTime;
		timeTaken = Mathf.Round(timeTaken * 1000.0f) * 0.001f;

		textBox.text = "" + timeTaken;


$$$$$$The Video$$$$$$

You can use ToString() to specify formatting. No need to mess with multiplication or division.

textBox.text = timeTaken.ToString ("0.000");