Amendment of 11 spreadsheets

I have 11 spreadsheets containing all of the postcodes in the UK. I require 3 postcodes from each area with all the others then being deleted. For example, there may be 1000 postcodes in BD23 area so BD23 1EA, BD23 1AB, BD23 1AA and so on. I require 3 postcodes from BD23 and every other area on the list so 3 from BD1, 3 from BD2 and so on. This needs to be done for all 11 spreadsheets. It can simply be done by going down each spreadsheet and deleting values, keeping 3 from each area. There may be a quicker way of doing it but doing the way I have described, I estimate it will be around 10 hours to complete the full set.