Climate change, biodiversity loss and land desertification are three of the most serious global environmental challenges faced by humans. As these challenges involve global public goods, it is crucially important that the global society should strive to make synergies between the three most influential United Nations (UN) conventions, which are the UN Framework Convention on Climate Change (UNFCCC), Convention on Biodiversity and the UN Convention to Combat Desertification (UNCCD). This paper reviews the negotiation process under the three conventions, especially focusing on cross-boundary issues. By analyzing the focal concerns of main parties, this paper evaluates possible future scenarios on cross-boundary issues under the three conventions. The cross-boundary issues can be summarized in the following aspects: mitigation, adaptation, loss and damage, and strategic plans and initiatives. It will promote synergistic actions under three conventions based on the ecosystem approach. In the end, we propose the following strategies: (1) lead the negotiations of the crossboundary issues initiatively; (2) construct and perfect the synergetic fulfillment mechanism of China; (3) improve the regional cooperation combining South-South Collaboration and the Belt and Road.