Do you want the number to be 001, or to display as 001?
For the first option you would need to convert it to text -- RIGHT("000" & Int((999 * Rnd) + 0),3)
For the second, go to the textbox or to the table field and set the format as 000. Note, however, that 001 will actually be 1 wehn you run a search for it.
Denis