Edit for special filter and count...

Ok,,, I've got a spreadsheet with over 60,000 entries each with a different post code.

I want to filter and then count to find the most re-occuring post code.

However, I want to do this just on postal area, so I need to edit the info so the last 3 characters of the post code are removed, eg...

SE10 4BX

becomes

SE10

other than doing it manually is there anyway to just command excel to delete last 3 characters from my list?