'Notepad++ Insert a character before the last three characters in every line

Is it possible and how do I insert a character before the last three characters in every line?

For example, I have something like this:

And I need it like this:

5.#400
#830
#400
1.#740
1.#000
2.#600
#310
2.#280
#830
#520
4.#650
1.#450
#930
5.#000

notepad++

Solution 1:^[1]

Explanation:

(?=     # positive lookahead, make sure we have after:
    ...     # 3 any character but newline
    $       # end of line
)       # end lookahead

Screenshot (before):

Screenshot (after):

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Solution	Source
Solution 1	Toto