You can add extra share buttons by filter hook as following

function post_grid_share_buttons_20200614($share_buttons){

  $share_buttons_extra = '
    <span class="linkedin">
      <a target="_blank" href="https://www.linkedin.com/shareArticle?mini=true&url='.get_permalink().'&title=&summary=&source="> </a>
    </span>';

  return $share_buttons.$share_buttons_extra;
  }

add_filter('post_grid_share_buttons','post_grid_share_buttons_20200614');

 

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