The three economic systems are the traditional, command and market economic systems. Different countries use different economic systems.
The traditional economic system exists in the Middle East, Asia, Africa and Latin America. It is based on every person's career being determined by birth. Children will probably do what their parents did to earn a living.
The command economic system gives individual citizens little say in the economy. Countries that use this type of economy include China, Russia and Uganda.
The market economic system says that what each person does is based on activity in the market and that individuals can decide for themselves how to make their money. The United States is an example of a market economy.