gistfile1.txt

Some notes here~

{% if relatedArticlesSelect | length == 3 %}
{% set relatedArticles = entry.relatedArticlesSelect.limit(3).all() %}

is the same as

{% set relatedArticlesSelect = entry.relatedArticlesSelect.all() %}

if you know there are only going to be three elements. dot all will fetch all of the elements attached

This is how I would code it ~

{% set selectedEntries = entry.relatedArticlesSelect %}
{% set existingEntries = ['not']|merge(selectedEntries.ids()) %}
{% set relatedEntries = craft.entries.section('articles').relatedTo(mainCategory).id(existingEntries).limit(3 - selectedEntries|length).all() %}
{% set allEntries = selectedEntries | merge(relatedEntries) %}

I believe that the sort order is post date desc by default, it might be asc.

This will hide elements that have already been output

This also assumes that the selected entries field is limited to three entries

Let me know if you have questions

Thanks, i've added an allEntries variable so it is more clear :)