YEREVAN, SEPTEMBER 6, ARMENPRESS/BTA. Deputy Prime Minister and Foreign Minister Mariya Gabriel headed the Bulgarian delegation to the Three Seas Initiative (3SI) Summit here on September 6 and 7, her ministry said in a press release on Thursday. Attending were the leaders of the twelve participating countries, and the key topics on the agenda were economic development, strengthening regional security, improving regional connectivity, and mobilizing the necessary investments for infrastructure projects of regional importance.