If the field is already a string, this will work
SELECT RIGHT('000'+ISNULL(field,''),3)
If you want nulls to show as ‘000’
It might be an integer — then you would want
SELECT RIGHT('000'+CAST(field AS VARCHAR(3)),3)
As required by the question this answer only works if the length <= 3, if you want something larger you need to change the string constant and the two integer constants to the width needed. eg
'0000' and VARCHAR(4)),4