Line 9x9" pan with foil, leaving a 2" overhang on two sides. Coat the foil with cooking spray.
Place 2 cups milk chocolate chips and 1 can condensed milk in a large microwave safe bowl and heat, stirring every 30 seconds, until melted and smooth, about 2 minutes. Transfer mixture to prepared pan.
Place the white chocolate chips, peanut butter chips and remaining condensed milk in a clean large microwave safe bowl and heat, stirring every 30 seconds, until melted and smooth. Spread the white chocolate mixture evenly over the chocolate mixture.
Place the remaining 1/2 cup milk chocolate chips and vegetable oil in a small microwave safe bowl and heat, stirring every 20 seconds, until melted and smooth.
Drizzle the chocolate mixture over the fudge, then refrigerate until firm, about 1 hour. Using the foil overhangs, lift the fudge from the pan and transfer from the pan to a cutting board. Cut into 25 squares. Store in an airtight container up to 1 week.