Do we need to place comma between all elements of a series including before "and" i.e three or more items of equal importance? For e.g. "suburban living is safer, less expensive, and more relaxing than city living."

Does the comma before "and" indicate that an independent clause has to come after "and"? What if we dont put comma before "and"? What rules says?

In the real world, you can choose whether to put a comma right before the and in a list of three or more things.

On the GMAT, that comma will be there. They don't test you, however, on whether to put the comma there or not, because it is not a "hard and fast" rule - they have just chosen to standardize on using the comma.