Even though this is the most straightforward rule for comma use, there is some contention about it, and it gives us a great opportunity to discuss lists in writing.
The Rule: Use a comma in between items on a list when there are three or more items.
Essentially, anytime you are using a conjunction like "and," "or," or "nor" and you have more than three things you are discussing, put a comma in between them.
"Early to bed and early to rise makes a man healthy, wealthy, and wise."
Our items in that list are: healthy, wealthy, wise
Our conjunction is: and
Which means the items preceding the conjunction get commas after them!
A Note About the Oxford Comma Debate: the comma after the penultimate (second to last) item is called the Oxford Comma, and some style guides do not use it. I come from the world of MLA which does use the Oxford Comma, so I will use it throughout my writing. You should check your style guide for your department or field and follow their standards.
Rules are Meant to be Broken
Sometimes when it comes to lists, you may want to play with the form to have a persuasive (rhetorical) affect on your readers. There are two simple ways to break this rule for dramatic affect:
Use a conjunction in between every item on the list. The greek rhetorical term for this is polysyndeton.
Don't use a conjunction between any items on the list. The greek rhetorical term for this is asyndeton.
If we take Benjamin Franklin's saying and tweak it to use more conjunctions, it reads: Early to bed and early to rise makes a man healthy and wealthy and wise.
And if we tweak to use no conjunctions: Early to bed and early to rise makes a man healthy, wealthy, wise.
Do you see how those feel different to you? The first has an energy, like each successive item is a surprise. The list should have ended at healthy, but then the writer remembered wealthy! And then they remembered wise!
The second rewrite feels more grounded and confident. The writer seems to be listing things in no particular order with confidence.
Once you understand the rules, you can break them. Exercise restraint though! Even though you can break the rules, doesn't mean you always should. Be intentional about when you use polysyndeton or asyndeton, and always have a clear goal for how you want to affect your readers.
Still Feel Stumped?
Let me give your writing a quick read over to double check your commas (and spelling and other punctuation)!