The design of a 3rd floor platform garden involves creating an outdoor space on an elevated area of a building. It typically requires considerations for structural support, soil management, water drainage, and plant selection to ensure the garden's sustainability and aesthetic appeal. Key aspects include choosing plants that can thrive in the specific conditions of the elevated space, considering the building's orientation for natural light and wind exposure, and incorporating elements such as walkways, seating, and decorative elements to enhance the garden's usability and visual appeal.