How can I filter post with only some categories?

Help
1

I am working on a page where I only want to show posts from certain categories

I have done this, but it works only with 2 categories, not with 3, aka you can’t use another or in the where_exp. anyone has an idea?

  <ul>
{% assign posts=site.posts | where: "lang", page.lang | where_exp: "post", "post.categories contains 'CAT1' or post.categories contains 'CAT2" %}
{% for post in posts %}
<li><a href="{{ post.url }}">{{ post.title }}</a></li>
{% endfor %}
</ul>