Pad leading zeros in a string.Format function

Question

How could use the string.Format function and in the format text pad a leading zero? 
Pseudo code is:
string parm = “5”;
string format = “Some number formatted as 3 dig: Format({0}, 000)”;
string output = string.Format(format, parm);
Where the output would look like this:
“Some number formatted as 3 dig: 005”
Thanks.

Answer 1

Thanks everyone. Unfortuantly there's a constraint where the padding operation needs to be embedded in the format string. This entire operatin is being put together dynamically where the format strings are being pulled from a library of format strings stored
in a database. at runtime we don't know how which format will be used and how many parameters will be passed in. we have logic to handle the unknown number of paramters as there is metadata which helps us with that.
the need for leading zeros needs to be part of the overall format defined in the format string.  for example, a real format string stored in the database looks like this:
"LINE_NO = '{0}',  AND GEO_LOCATION = 'T{1}, {2},  R{3}, {4}, , Sec. {5}'"
and its output from the string.Function is:
"LINE_NO = 'TG-G2469',  AND GEO_LOCATION = 'T155, N,  R93, W, , Sec. 5'"
the last parameter (param 5) had a value of 5.  In another case the output requirement might need to look like this:
"LINE_NO = 'TG-G2469',  AND GEO_LOCATION = 'T155, N,  R93, W, , Sec. 05'"
note the "05" at the end...
the logic of using the leading zero needs to be embedded in the overall format string text.
Thank you.