{% assign random1 = site.time | date: "%s%N" | modulo: site.posts.size %}
{% assign post_random1 = site.posts[random1] %}
{% assign random2 = site.time | date: "%s%N" | plus: 20 | modulo: site.posts.size %}
{% assign post_random2 = site.posts[random2] %}
{% assign random3 = site.time | date: "%s%N" | plus: 80 | modulo: site.posts.size %}
{% assign post_random3 = site.posts[random3] %}

<h2>Random Posts</h2>
<ul>
  <li><a href="{{ post_random1.url }}">{{ post_random1.title }}</a></li>
  <li><a href="{{ post_random2.url }}">{{ post_random2.title }}</a></li>
  <li><a href="{{ post_random3.url }}">{{ post_random3.title }}</a></li>
</ul>