how to achieve Buy 2 products in some category and get any 1 product in same category for free

Question

say for eg in same category

Buy A and B then C product should be free or 2A then B should be free

Answer 1

Finally I did it by myself so these are the steps I followed to achieve it

1. I am using magento version 2.1.6. first I've created a new cart price rule find it in below image

2. Since I am using event "salesrule_validator_process" so in module etc/events.xml I wrote like this

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
     <event name="salesrule_validator_process">
        <observer name="custom_cart_rules" instance="Almusbah\SendSms\Observer\CustomCartPriceRules" />
    </event>
</config>

3. Finally Observer file "CustomCartPriceRules.php" does the magic

   <?php
   
   namespace Almusbah\SendSms\Observer;
   
   use Magento\Framework\Event\ObserverInterface;
   use Magento\Framework\App\ObjectManager;
   
   class CustomCartPriceRules implements ObserverInterface {
       public function __construct() {
   
       }
   
       /**
        *
        * @param \Magento\Framework\Event\Observer $observer
        * @return void
        */
       public function execute(\Magento\Framework\Event\Observer $observer) {
   
           $result = $observer->getEvent()->getResult();
           $item = $observer->getEvent()->getItem();
           $rule = $observer->getEvent()->getRule();
           $address = $observer->getEvent()->getAddress();
           $qty = $item->getQty();
   
           if ($rule->getData('name') == 'Buy 2 get 1 free'):
   
                   // overriding magento rules here and updating Almusbah Offer
                   $result->setAmount(0)
                       ->setBaseAmount(0)
                       ->setOriginalAmount(0)
                       ->setBaseOriginalAmount(0);
   
                   $item->setDiscountPercent(0);
   
   
               $products = [];
               $z = 0;
               $totalQty = 0;
               $dA = 0;
               foreach ($address->getAllItems() as $q):
   
                   $validate = $rule->getActions()->validate($q);
   
                   if ($validate):
                       $products[$z]['sku'] = $q->getData('sku');
                       $products[$z]['qty'] = $q->getData('qty');
                       $totalQty += $q->getData('qty');
                       $products[$z]['totalQty'] = $totalQty;
                       $freeQty = (int) ($totalQty / 3);
                       $products[$z]['freeQty'] = $freeQty;
                       if ($products[$z]['freeQty'] > 0):
                           $products[$z]['applyDiscount'] = 1;
                           $dicountPercent = 100;
                           $dA = ($products[$z]['freeQty'] * $q->getData('calculation_price'));
                           $products[$z]['discountAmount'] = $dA;
                           $totalQty -= 3*$products[$z]['freeQty']; // appying on each 3 items
                       endif;
                       $z++;
                   endif;
               endforeach;
               //echo '<pre>';print_r($products);
               $chkCurrentSku = $item->getSku();
   
               foreach ($products as $p):
                   if(isset($p['applyDiscount']) && !empty($p['applyDiscount']) && $chkCurrentSku == $p['sku']):
                       $discountAmount = $p['discountAmount'];
                       $fullDiscount = 100;
                       $item->setDiscountPercent($fullDiscount);
                       $this->almusbahBuy2Get1Offer($discountAmount,$result);
                   endif;
               endforeach;
   
           endif;
   
   
       }
   
       public function almusbahBuy2Get1Offer($discountAmount, $result) {
   
                      $result->setAmount($discountAmount)
                           ->setBaseAmount($discountAmount)
                           ->setOriginalAmount($discountAmount)
                           ->setBaseOriginalAmount($discountAmount);
               }
   
       }
   

   ?>

4. That's it don't forget to update the module using

   php bin/magento setup:upgrade