The first countries to have flags were countries of ancient China that later were united into modern China. What one can say is that the first flags were in China.

The similarity of the three flags you mentioned has historical rational. All three derive from the flag of the United Provinces of Central America. That flag was adopted when the country formed in 1822, and in turn was based on the Argentinian flag because Argentina had sent an expeditionary force to help the Central Americans rebel against the Spanish. The United Provinces broke up in 1838; but, three of its five countries eventually chose flags derived from its flag.
Source(s):
my mains source is an actual book:
The World Encyclopedia of Flags by Alfred Znamierowski
http://www.amazon.com/World-Encyclopedia-Flags-Alfred-Znamierowski/dp/07548...
which I strongly recommend if you are interested in flags.

http://en.wikipedia.org/wiki/United_Provinces_of_Central_America

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