El Salvador, Honduras, and Nicaragua
Costa Rica, El Salvador, Guatemala, Honduras, and Nicaragua declared independence from Spain on September 15, 1821. The countries were then part of the First Mexican Empire until the monarchy was overthrown in 1823. The five countries became part of the United Provinces of Central America in 1823, and they adopted one flag similar to Argentina’s (Panama was part of the Republic of Colombia, and Belize was a British colony).
The United Provinces of Central America began to dissolve in 1838, and El Salvador, Honduras, and Nicaragua ended up with flags similar to the one they had when they were one province.
How to remember which is which: They each represent the five nations in one way or another. Honduras does it with stars. Nicaragua, which is the most popular volcano boarding spot (more on that later), does it with volcanoes. And El Salvador has five flags and volcanoes; basically, the flags become more elaborate as the names get longer.