Many people were taught that we use commas before a conjunction in a series as in:

This, that, and the other thing.

But that's not really the accepted use of commas in a SIMPLE SERIES.

For comma use in a series, we follow the AP Stylebook:

In a Series:

Use commas to separate elements in a series, but do not put a comma before a conjunction in a simple series:

The flag is red, white and blue. He would nominate Tom, Dick or Harry.

Put a comma before the concluding conjuction is a series, however, if an integral element of the series requires a conjuction:

I had orange juice, toast, and ham and eggs for breakfast.

Use a comma also before the concluding conjunction in a complex series of phrases:

The main points to consider are whether the athletes are skillful enough to compete, whether they have the stamina to endure the training, and whether they have the proper mental attitude.