From the Document: "The Three Seas Initiative (3SI) is a regional effort in Europe to expand cross-border energy, transportation, and digital infrastructure and boost economic development in the area between the Adriatic Sea, Baltic Sea, and Black Sea. Twelve countries (Austria, Bulgaria, Croatia, the Czech Republic, Estonia, Hungary, Latvia, Lithuania, Poland, Romania, Slovakia, and Slovenia), all of which are members of the European Union (EU), participate in the 3SI. [...] The U.S. government, including some Members of Congress, have indicated strong backing for the 3SI. In November 2020, the House of Representatives passed a bipartisan resolution 'expressing support of the Three Seas Initiative in its efforts to increase energy independence and infrastructure connectivity thereby strengthening the United States and European national security' (H.Res. 672, 116th Congress). Officials from the Biden Administration and the Trump Administration have expressed diplomatic support for the 3SI, and high-level U.S. officials (including then-President Trump in 2017) have joined its annual summits."
CRS In Focus, IF11547
Congressional Research Service: https://crsreports.congress.gov/