The fourth World Summit on Sustainable Development in Johannesburg, South Africa, is seen as a key platform for issues such as poverty and tackling the ecological cost of mining and deforestation.
Delegates will be focused on creating an action plan for Agenda 21, the sustainable development concept drawn up at the Rio Earth Summit in 1992.
Yasmin Tariq from Conwy, Natasha Parry from Flint and Rhys Richardson from Cardiff, won their places to Johannesburg with a project on sustainable development at the Centre for Alternative Technology in Machynlleth.
They are part of an assembly-led educational project, giving young people a voice on the future of their environment.
The group has spent weeks researching its case for a sustainable Wales of the future, first presented at the New Future for Wales conference, held in Aberystwyth in July.
An estimated 60,000 people from dozens of countries will be attending.
First Minister Rhodri Morgan will be among leaders present in South Africa from around the world to hear the case for sustainable development.
He will meet the Welsh delegates and is likely to be pressed for his thoughts on a sustainable public transport service across Wales.
Mr Morgan - who is taking three officials with him - said Wales had "valuable lessons" to share with other countries about sustainable development.
"We also know all about the alternative - unsustainable development if you like," he said.
"Our communities and landscape still bears the scars of that kind of development, which didn't consider the long term future.
"We have to learn to live differently, and we have a key role to play in sharing the lessons we have already learned with other nations."
Mr Morgan is sponsoring an event at the summit and is due to give a speech on corporate trust and responsibility and meet representatives of other regional governments.