Q:

How do I change two letters closest to a string and one letter immediately after a string using Notepad++?

I have a list of emails, and I want to change the two letters before "@" and the first letter after "@" using Notepad++.
For example:
username@yourdomain.com

becomes
userna**@*ourdomain.com

A:

I want to change the two letters before "@" and the first letter after "@"

Menu "Search" > "Replace" (or Ctrl + H)
Set "Find what" to ..@.
Set "Replace with" to **@*
Enable "Regular expression"
Click "Replace All"

Before:
username@yourdomain.com

After:
userna**@*ourdomain.com

Further reading

How to use regular expressions in Notepad++ (tutorial)
Notepad++: A guide to using regular expressions and extended search mode
Regular Expressions Tutorial
RegExr: Learn, Build, & Test RegEx
regex101: Online regex tester and debugger
RegExper: Regular Expression Visualiser

A:

You can do this by using a regex search/replace.
At the bottom, select Regular Expression.
In the Search for entry, you type in: ..@.
In the Replace with, you type in **@*
Then press the button Replace All
This works because Regex searches will only replace if its search criteria matches exactly. The match is explained as follows:
..@. There are 3 dots and an @:

@ has no special meaning in regex so it means a literal @.  
. means any character, exactly once. By writing .. it means 2 characters of any kind, as long as there are 2 characters.

