Hi Vetri,

The requirement is not straight forward and would require customizations. I will help you with the design, and then may be you can start building the required objects etc.

Design Approach - 
1. Custom Object (Product Discounts) - Create a new object that stores all possible combinations of products that can exist for an opportunity with the respective discounts. Use a multi-select picklist field for storing the Product Combinations.
2. On Creating a new Line Item record a piece of code should do the following - 
       - Query the Custom object with the product combination present on the respective opportunity including this newly created Product
       - If the query returns a record then use the discount value mentioned in that record and update the opportunity Total Discount field
Example: If an Opportunity already has two line items with Products A and B respectively. When the user creates another line item with Product = C, the code should check if the Custom Object contains a record with the combination of A, B and C. If yes then the respective discount value should be used to update the Total Discount field on Opportunity.

Approach for checking Product Combination - 
1. Use a multi-select picklist field for storing the Product Combinations and another field to store the discounts.
2. When a new line item is created, create a string containing all the Products for that opportunity delimited with a semicolon ';'
3. This should return a single record of the Custom Object that matches the same combination of Products
4. Use the discount value of the retrieved record and update the Total Discount field of Opportunity

Point to Consider - 
You can choose between a Trigger and the new Process Builder for invoking this piece of code when a new line item record is created.

Sample lines of code to compare the combination of products with the Custom Object - 

//--- For Loop that traverses all the line item records for an opportunity and creates a ; delimited string of all Products
For (OpportunityLineItem oLI : [select Product2 from OpportunityLineItem 
                                where OpportunityId = :ParentId]) {
     //--- Declare ProductList as a String outside this For Loop
     if (ProductList == '')
         ProductList = oLI.Product2;
     else
         ProductList = ProductList + ';' + oLI.Product2;
}

//--- Product_Discounts__c is the new Custom Object created to hold the discount information
//--- MultiSelect_1__c is the multi select picklist
//--- Discount__c is the field to hold the Discount for the Product Combination
//--- This query checks the combination of line items with the Custom Object records. The order of Products in ProductList can be different than MultiSelect_1__c, it does not matter.
List<Product_Discounts__c> pDis = [select Id, MultiSelect_1__c, Discount__c from Product_Discounts__c 
                     where MultiSelect_1__c = :ProductList];

if(pDis.size() > 0)
	System.debug('Found a record with Discount equal to ' + pDis[0].Discount__c);

I have tried to be elaborative in explanation. I hope this solves your problem.

Thanks
AR

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