Here are the suggested answers for the blanks in the text:

1. **(3) goods** - "people used to exchange goods."
2. **(4) disadvantages** - "has many disadvantages."
3. **(5) smaller** - "may be impossible to divide into smaller units."

The completed passage would read:

"In the earliest periods of human history, people used to exchange goods. They would exchange things they had plenty of for things that they were in need of. For example, they might offer food for tools. This method of exchange, which is known as barter, has many disadvantages. Certain goods may be difficult to carry, they may not last long, or may be impossible to divide into smaller units. It can also be difficult to know the worth of something compared with other goods."