In the following case, should we add a comma before "or"?
I will eat apple, orange or banana.
I will eat apple, orange, or banana.
The comma is completely optional. It's called the serial or Oxford comma and its use it a matter of style. You are correct with it, or without it. It's important to be consistent, and either always use it, or always not use it, without changing from one to the other.
However, I'll be stronger than buggles in the comment about the articles. Without them, the sentence is incorrect.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.