The meeting will be held at Kórnik Castle in Poland's western Wielkopolskie region on May 12, Poland's European Affairs Minister Szymon Szynkowski vel Sęk has said.
Ukrainian Deputy Prime Minister Olha Stefanishyna, who is in charge of her country's bid to join the EU and NATO, is also expected to attend the meeting.
"The main topic will be EU enlargement, with a particular emphasis on Ukraine," Szynkowski vel Sęk told reporters.
"We will also talk about the European perspective of Moldova and the Western Balkans," he added.
The three ministers will also discuss ways to reform the EU's institutions, according to Szynkowski vel Sęk.
The Weimar Triangle group was set up by Poland, Germany and France in the early 1990s to help overcome divisions in Europe by integrating Poland with European and Euro-Atlantic structures. That goal was achieved through Poland's accession to NATO, and then to the European Union.
Today the Weimar Triangle is a platform for consulting and discussing common positions on key policy issues within the European bloc, with talks taking place at various levels of government administration, according to officials.