How to select lines from text by parameters?

Question

Notepad++

Hello, help.
Text 5k lines, in alphabetical order. You need to save the first 3 lines of each letter of the alphabet. Given the first 2 characters of the string.
It should look like this
: at58
at59
at60
au344
au347
au348
ba5
ba6
ba7
be72
be73
be74

Answer 1

https://regex101.com/r/SgMRfT/1
Find: (?m-s)^(..).*+(?>\r?\n\1.*){2}\K(?:\r?\n\1.*)+
And leave the replacement string empty.