Yes, you use a comma before "and" in a series of three or more words or phrases.
Example:
* I bought apples, oranges, and bananas at the market.
Why?
The comma helps to clarify the structure of the sentence and avoids ambiguity. Without the comma, it might be unclear whether "oranges and bananas" are a single item or two separate items.
Exceptions:
* Short, closely related phrases: You may omit the comma if the phrases are very short and closely connected, like "bread and butter."
* Technical or legal writing: Some styles, like technical writing, might omit the comma before "and" to avoid confusion.
In general, it's safer to err on the side of using the comma before "and" in a series of three or more words or phrases. This will ensure your writing is clear and grammatically correct.