may be this code help you.....
string empID = "";
for (int i = 0; i < 1000; i++)
{
if (i.ToString().Length == 1)
{
empID = "EMP00" + i.ToString();
}
else if (i.ToString().Length == 2)
{
empID = "EMP0" + i.ToString();
}
else if (i.ToString().Length == 3)
{
empID = "EMP" + i.ToString();
}
listBox1.Items.Add(empID);
}