Change number of blog columns via media query?

The problem on that is that enfold organizes the entries (artikles) on a grid layout by column setting.
See here example page of enfold grid layout blog: https://kriesi.at/themes/enfold-2017/blog/blog-default/
if you take a look on developer tools you see that on a 3column grid groups of 3 entries are build to have that layout:
in a three-column grid layout, 3 entries each have their own wrapper

(click to enlarge the image)

so if you have the chance to handle the whole grid as one wrapper and have some media queries to style them you had to bring all entries to one wrapper
put this to your child-theme functions.php

function wrap_articles_in_one_flex_container(){ 
?>
<script type="text/javascript">
window.addEventListener("DOMContentLoaded", function () { 
  (function($) {
    $('.avia-content-slider-inner .slide-entry-wrap .slide-entry').each( function() {
        $(this).detach().appendTo('.avia-content-slider-inner .slide-entry-wrap:first');
    });
    $('.avia-content-slider-inner .slide-entry-wrap:empty').remove();
  })(jQuery);
});
</script>
<?php
}
add_action('wp_footer', 'wrap_articles_in_one_flex_container');

And then style that one wrapper f.e. with flexbox layout on quick css:

#top .avia-content-slider .slide-entry-wrap {
  display: flex;
  flex-flow: row wrap;
  justify-content: space-between;
}

#top.blog .slide-entry::before, 
#top.blog .slide-entry::after {
  display: none;
}

#top .avia-content-slider .slide-entry-wrap .slide-entry {
  flex: 0 1 calc(33% - 30px) ;
  margin: 0px 15px 30px !important;
  width: unset !important;
  padding: 0px !important;
}

@media only screen and (max-width:989px) {
  #top .avia-content-slider .slide-entry-wrap .slide-entry {
    flex: 0 1 calc(50% - 30px);
    margin: 0px 15px 30px !important;
  }
}

@media only screen and (max-width:767px) {
  #top .avia-content-slider .slide-entry-wrap .slide-entry {
    flex: 0 1 100%;
    margin-bottom: 30px !important;
  }
}

see here : https://webers-testseite.de/blog/

One thing to mention: a paginated blog will not work correctly that way – because the amount of entries each page will not be influenced.

PS: On the example page of enfold there is an audio entry – and on this one the width calculation does not work because of a setting by Enfold. On this page it would be necessary to set the width:

#top .avia-content-slider .slide-entry-wrap .slide-entry .mejs-container {
  min-width: unset !important;
  width: unset !important;
}