What is the most appropriate logic to add rows in table

I have following table structure (simplified to make essential things clear), which lists Top 3 bank customers in each category of loan and branch of the bank. SNO column is rank of the customer, value of which is up to 3.

Loan Category	SNO	Branch	Customer Name	Amount
Home Loan	1	abc	Piyush	10000
Home Loan	2	abc	Shyam	5000
Home Loan	3	abc	Kamal	2000
Home Loan	1	xyz	Xman	50000
Home Loan	2	xyz	Shyam	20000
Auto Loan	1	abc	Birendra	10000
Personal Loan	1	xyz	Gyan	5000
Personal Loan	2	xyz	Prakash	2000

I am trying to make another table such that, If there are less than 3 customers in each loan category and branch, Insert a dummy row for each branch and category with values of customer name and amount as NULL. Essentially, I am trying to get following table.

Loan Category	SNO	Branch	Customer Name	Amount
Home Loan	1	abc	Piyush	10000
Home Loan	2	abc	Shyam	5000
Home Loan	3	abc	Kamal	2000
Home Loan	1	xyz	Xman	50000
Home Loan	2	xyz	Shyam	20000
Home Loan	3	xyz			added row
Auto Loan	1	abc	Birendra	10000
Auto Loan	2	abc			added row
Auto Loan	3	abc			added row
Auto Loan	1	xyz			added row
Auto Loan	2	xyz			added row
Auto Loan	3	xyz			added row
Personal Loan	1	xyz	Gyan	5000
Personal Loan	2	xyz	Prakash	2000
Personal Loan	3	xyz			added row
Personal Loan	1	abc			added row
Personal Loan	2	abc			added row
Personal Loan	3	abc			added row

I have solved this problem by using Loop iterating over all category and branch and inserting dummy row, if max(sno) < 3 for each category/branch. But, I am looking for appropriate logic without iterating over all category and branch. In my actual table, there are thousands of branch values and more than 100 categories. So, iterating over all combination of category and branch is very expensive in terms of performance.

I need to write some good logic preferably using SQL constructs only or not using any loop.

Answer

Ok. So you must have some tables where branch and category is listed in single or multiple tables. Lets take it as your branch and category tables and you must have some query which produced the result mentioned in the question. Lets call it your_query.

You need to generate 3 records per branch per category.

Select c.category as loan_category,
       L.lvl as sno, 
       B.branch,
       Q.cutomername,
       Q.amount
  From category c 
  Cross join branch b
  Cross Join (select level lvl from dual connect by level <= 3) l 
  Left Join your_query q on q.branch = c.branch 
                        and q.category = c.category
                        and l.lvl = q.sno
 Ordet by c.category, B.branch, L.lvl