<?php

class TimeSlot {
  private $posts = [];

  public function isAvailable();
  public function publish(Post $post) {
    $this->posts[] = $post;
    $post->onPublish($this);
  }
}

class Post {
  private $publicationSlots = [];
  public function onPublish(TimeSlot $timeSlot) {
    $this->publicationSlots[] = $timeSlot;
  }

  public function canBePublished() {
    return count($this->publicationSlots) < 3;
  }
}


$timeSlot = $repo->findForDate($dateTime);
$timeSlot->publish($post);
$repo->save($timeSlot);