Finally After 3 Days i Found The Solution :

1/ i created a new script

2/attached it to the 3d Text

3/Add code to change the text:

time-= Time.deltatime; // CountingDown GetComponent<TextMesh>().text = time.ToString("0");// Show the countdown inn the 3d text

Finally! After 3 days, I found the solution:

1. I created a new script

2. attached it to the 3d Text

3. Add code to change the text:

    time-= Time.deltatime; // CountingDown
    GetComponent<TextMesh>().text = time.ToString("0"); // Show the countdown in the 3d text