The trio refers to the 3 EU countries which hold the presidency of the Council of the European Union for a total of 18 months and which work closely together over that period. Each member of the trio chairs all configurations of the Council for a 6-month period, with the exception of the Foreign Affairs configuration.
This system was introduced after the Lisbon Treaty in 2009. Each trio sets long-term goals and develops a joint agenda identifying the topics and issues to be addressed by the Council over a period of 18 months. Each of the 3 EU countries prepares its own more detailed 6-month programme, taking due account of the goals and the agenda.
Source: EUR Lex Glossary