Organisers said the event would now take place in the Estonian capital, Tallinn, on October 19 and 20.
The summit and forum will bring together heads of state, leading business figures and senior officials from Europe and the United States to discuss the economies of Central and Eastern Europe and energy security, the organisers said in a statement.
The Polish-led Three Seas Initiative aims to boost infrastructure, energy and economic cooperation among 12 countries between the Black, Baltic and Adriatic Seas.
The initiative brings together Austria, Bulgaria, Croatia, the Czech Republic, Estonia, Hungary, Latvia, Lithuania, Poland, Romania, Slovakia and Slovenia.
Its key partners are Germany, the European Union and the United States.