You will need two fields. One field where you enter the number. And a calculation field that adds the leading zeroes.
Let's say you have a field called "Number" as a number field where you enter your number.
You then make a calculation field called "c_Number_Zeroes" (for instance) where you create the following calculation:
Right ( "000" & Number ; 3 )
This means you will be combining "000" with your number.
12 will become 00012.
1 will become 0001
235 will become 000235.
But the Right function takes only 3 characters from the right side.
So you will always end up with 3 characters and leading zeroes.
Thanks - obvious when you think about it !!
A text field with an auto-enter calculation could do this with a single filed instead of two--though that may not be a better option in all cases:
Right ( "000" & GetAsNumber ( self ) ; 3 )
Make sure to clear the "do not replace existing value..." check box.