3SI was launched by Poland and Croatia in 2015. Currently, it includes 13 EU member states (Austria, Bulgaria, Croatia, Czechia, Estonia, Lithuania, Latvia, Poland, Romania, Slovakia, Slovenia, Hungary, and Greece) located between the Adriatic, Baltic, and Black Seas. 

Ukraine, Moldova, Albania, and Montenegro are associated participating states to 3SI. The strategic partners of 3SI are the USA, the European Commission, Germany, Japan, Turkey, Spain, and Italy.

So far, 11 3SI summits at the level of heads of state and government have been organized – in Dubrovnik (2016), Warsaw (2017), Bucharest (2018), Ljubljana (2019), Tallinn (2020), Sofia (2021), Riga (2022), Bucharest (2023), Vilnius (2024), Warsaw (2025), and Dubrovnik (2026). There have also been 8 Business Forums accompanying the Summits since 2018, bringing together representatives from the business community. The next Summit and Business Forum will take place in the first half of 2027 in Slovakia, who currently holds the informal presidency in 3SI. 

3SI is an informal platform for regional cooperation. The only institutionalized form of cooperation remains the Initiative's investment vehicles:

• Three Seas Initiative Investment Fund is an investment vehicle used to provide capital financing for key infrastructure projects in the Three Seas region.
• Three Seas Initiative Innovation Fund is an investment vehicle operating as a fund of funds, aimed at supporting growth-stage companies in Central and Eastern Europe.

On April 28, 2026, on the sidelines of the 11th Summit and 8th 3SI Business Forum in Dubrovnik, Bank Gospodarstwa Krajowego (BGK) and the national development banks of Croatia (HBOR), Romania (BID), Lithuania (ILTE), and Slovenia (SID Banka) signed a Memorandum of Understanding on the creation of the Three Seas Initiative Infrastructure Fund of Funds, managed by the European Investment Fund (EIF). Its goal is to mobilize at least €2 billion (about 8.5 billion PLN) for infrastructure development in the 3SI region.