What is one similarity in how the Greeks, Romans and the US structured their government?

One similarity is that all three civilizations had a form of representative democracy. In ancient Greece, citizens participated in the governing of the city-state through direct democracy, where they voted on laws and policies. In ancient Rome, the Republic had a system of elected officials who represented the interests of the people. In the United States, the government is based on a system of representative democracy, where citizens elect representatives to make decisions on their behalf.