new count;

if(strcmp("/count", cmdtext, 6) == 0)
{
    count = SetTimer("Count", 1000, false);
    SendClientMessageToAll(COLOR_GREEN,"Countdown started");
    return 1;
}

forward Count();
public Count()
{
    new counter = 3, string[10];
    counter--;
    format(string, sizeof(string),"~g~%d", counter);
    GameTextForAll(1000, 3);
    if(counter == 0)
    {
        GameTextForAll(3000, 3, "~g~Gooooo!!!");
        KillTimer(count);
    }
    return 1;
}