This paper addresses the concept of sustainable urban design by examining three prominent theories: New Urbanism, Compact City, and Green Urbanism. It analyzes the application of these theories through three significant urban projects: Celebration, Florida (New Urbanism), Hammarby Sjöstad (Compact City), and Atlanta BeltLine (Green Urbanism), based on six core principles of sustainable urban design, including mixed-use integration, sustainable mobility, social equity, environmental efficiency, quality of life, and urban resilience. The analysis reveals that the application of these principles faces considerable challenges. For instance, Celebration demonstrates success in improving public spaces and supporting sustainable mobility but fails to address social equity and environmental responsiveness. Hammarby Sjöstad, on the other hand, integrates urban density with sustainable systems, but economic challenges remain in fully implementing environmental solutions. Atlanta BeltLine offers a strong example of how green urbanism can repurpose old infrastructure into green networks, although social challenges, such as real estate inflation, threaten its social equity goals. The paper concludes with a discussion on the gap between theory and practice, highlighting how social and economic factors play a significant role in limiting the realization of sustainable urban design goals. Finally, it emphasizes the need for collaboration between urban planners, designers, policymakers, and local communities to develop flexible design solutions that balance environmental, social, and economic considerations. This research underscores the complexity of sustainable urban design, emphasizing the importance of adapting theories to real-world challenges in order to create cities that are resilient, inclusive, and sustainable for future generations.