Any ideas how to write this function or where I might go for help?

I have a column of 5 digit zip codes and column of frequecy of those zip
codes.
I would like to find the frequency of the first three digits of zip codes
(***xx)
I have started by multiplying by.01 but cannot figure out how to keep the
zero as first digit.
I started ? manually but realized I would be here all night. Too nice a day
for that.

Thanks for any help.