For your first function, I am slightly confused why you put an if. You are not checking if cube is equal to number ** 3. I think you are just setting cube to equal number cubed and returning the result.
Solution: Remove if and replace with the word return
For your second function, I am really not understanding what you are trying to do. For example, you check if number divided by "by_three" is equal to zero. by_three is just the name of your function, so that doesn't do anything. It should be: if number % 3 == 0. The % sign gives the remainder of dividing the first number by the second number.
Another quick note is you return "cube(number)" which will just return the actual words, cube(number). I think you are trying to return the act of calling your first function which means you should remove the quotes.