3-phase core type transformers are one of most important equipment and are mostly used as power transformers in various applications. These transformers are equipped with a third winding for supplying station auxiliaries and for other reasons which is called tertiary winding. The cost of these transformer is a sizable proportion of the total system cost within which it is operated. A number of methodologies have already been developed to execute design of the transformer cost optimally. However, a developed a simplified method for optimizing the design process is depicted in this book in presence of constraints specified by the customer and the regulatory authorities, through exhaustive search using nested loops. The method appears to be best at present considering the enormous speed of the computers. A case study is given in this book based on this method and has been given at the end. Optimal design of 3-phase 3-winding transformers have been made by using two methods and the cost of production has been taken as the objective function based on the two key variables. Exhaustive search and sequential gradient search are used to reach the goal.